Contents
1. Overview
2. The LaTeX Editors (mostly for *nixes)
3. Windows(-only?) LaTeX Editors
4. MacOS Editors
5. A Parting Word on Typesetting versus Word Processing
Caveat: Check the date on this post, if it's years old then some of it might be out of date. Feel free to add comments and suggestions for improvement.
1. Overview
In the free software category emacs, kile, lyx, and texmaker have to be standout LaTeX/Tex/BibTeX editors for GNU/Linux, Unix and MacOS. For Windows I haven't seen anything better than LEd. Honorable mentions go to TeXMacs. I'll focus on GNU/Linux systems, but Windows and MacOS users can also find some guidance.
For the uninitiated: You never need to feel bound to any particular LaTeX editor (unless you're one of those fanatics who just LIVES within emacs or vi



2. The LaTeX Editors (mostly for *nixes)
Kile is the KDE LaTeX editor. I've found Kile to be great not only for editing raw LaTeX and BibTeX files and running the binaries but also for all the extra goodies, it makes latex editing feedback almost as good as wysiwyg word processing. It is incredibly flexible and powerful. I just add buttons to the toolbar to run my own wrapper scripts to do some really fancy stuff, such as fixing Index and Glossary files. (Glossary files seemed to not admit more than so many characters, so i made a script to automatically add the full glossary text that I needed and proces the tex source files from latex right through to PDF generation with one button click). LEd, the Windows free-LaTeX editor and viewer seems possibly more user-friendly than Kile, but since it's a native Windows application I can't run it under GNU/Linux, sadly (unless I install Wine). I'm not sure about LEd, but Kile has zero bondage, meaning any .tex files and .bib files that work with Kile should just work straight up in any other LaTeX aware environment (modulo dumb things like carriage returns when moving files from DOS to Unix), and believe me, this is a very good thing (zero bondage).
Now, if you are obsessive about wysiwyg then you can virtually have it with Kile, here's how: Simply create a toolbar button for running latex-dvi2ps-ps2pdf (or your favourte PDF generation sequence) and add a button for opening the pdf typeset document in Kpdf (as an external process, not embedded within Kile). Now everytime you run your pdf generator it'll immediately update the Kpddf window, so you'll see near instantaneous wysiwyg feedback. It's not perfect real-time viewing, but it's close enough. (Note Kghostview doesn't reload for instant updates when it's PS source is changed, so you can't stop at the dvi2ps step to make it faster for viewing, it's slightly quicker I think to generate the PDF and view that.) Kile also has a Viewer mode, so you can rapidly switch between a DVI (or PS) view and back to editing mode, which again is almost as good as wysiwyg feedback. Finally, if you don't care about viewing images then the DVI viewer gives really ultra-quick feedback, just run latex, that's all, and it instantly updates your DVI viewer (Kdvi by default of course, which instantly reloads when the dvi file changes). If you must clutter your screen with a wysiwyg view then this is almost as good as LEd and LyX. (Since an editing save is Ctrl-S and running latex is Alt-2 you are literally only ever two keystrokes away from a wysiwyg live update view of your document with Kile.) So Kile is my current favourite, though admittedly if i was a Windows user I'd be using LEd.
Texmaker is harder for me to evaluate since I've only read the overview and played with it for a few minutes. It is GPL licensed like Kile and Emacs. It is very similar to Kile in it's interface and features. Kile has the advantage, I think, of having the use of KParts, e.g., Kile will be network transparent, so you could work on files on remote machines seamlessly. One wonders why Texmaker and Kile both exist, which editor was released first? Well, from the looks of things I guess Texmaker is French in origin and was first released about the same time as Kile, whereas Kile seems to have a more German developer flavour (both projects seem to date from 2003). Both are excellent free software projects, but I favour Kile for extensibility and because of my bias in not having used Texmaker much (maybe texmaker is just as flexible, but I wouldn't bet on that).
Emacs---for fanatics. I'm still fond of emacs (and vi for that matter, see next entry) although I would advise newbies, especially *nix newbies to try LaTeX editing with a simpler point&click IDE rather than try beginning with emacs or vi, simply because these two editors require some dedicated time put in to learn the keyboard commands. Of course if you've committed to memorizing all the vi or emacs key command sequences then these editors may be the best for you, but just admit that you are now a bondaged slave!

vi/vim--- As with emacs, vi is incredibly powerful and has LaTeX aware extensions to make things really nice. But there are a whole lot of keyboard command controls that one has to memorize to be able to work efficiently on a large project. That's OK if you use VI a lot, in fact if you have the time to master vi then I would highly recommend it. I hate to lower my vi karma, but I have to suggest that for newbies vi is not so good, if only because it takes awhile to get used to working with vi. karma by pointing out that if you get deep into using vi for all your editing then it probably becomes the most powerful editor, or at least on par with Emacs for shear broad utility. If you're a vi expert and new to LaTeX then you should for sure start with vi as your LaTeX editor. See the comments below in the post below by Ted (commenting on an older version of these notes) which prompted me to revise this section to boost the rating of vi. Same comments therefore apply to vi/vim as for emacs---it's a kick-ass application if you're an expert user.
LyX---Does it Suck? I've worked with LyX a bit, but was not overwhelmingly impressed, that was 5 years ago, so it could be great now! In fact I had a small play with LyX just a minute ago, and it does look greatly improved. But how can it maintain it's wysiwyg interface integrity when you add arbitrary CTAN packages? I don't think it's possible, unless it runs latex in the background to generate the rendered typesetting? Hence, I believe that LyX suffers from a high degree of bondage and maybe even religiosity amongst it's users. Nevertheless it's a great tool to use if you are starting out from scratch. I probably won't ever know for sure because for me my custom preambles just were not handled at all well by LyX, so it was just too much of a chore to migrate to LyX. I find the speed of latex and bibtex on modern processors is fast enough that it's near enough to wysiwyg anyway! ergo, no need for LyX or KLyX (Klyx is a clone of LyX for KDE, so it should also be pretty sweet to use one would imagine). If I wanted a LaTex editor that shows me the rendered typesetting in real time then I'd be inclined to use LEd instead. For experienced (La)TeX users you just can't beat writing in direct LaTeX source code, and LyX just seems to make it harder to do that, so despite having a pretty interface, for speed a tool like LEd can't be beat.
There is one caveat to all this! If you are totally new to the world of (La)TeX then LyX is the way to go, at least initially. It'll give you 90% of the functionality of any laTeX editor but with wysiwyg feedback built-in. So it'll have the look of conventional word processing while also encouraging you to get used to content and structure in the spirit of TeX/LaTeX. Also, for including pictures in your document LyX is terrific because it gives you instant visual feedback on how the figure will appear, whereas with Texmaker, Kile, emacs, etc., you till have to run latex then view a DVI or PS file before you can see if the images turn out OK or not. While a minor inconvenience for experienced users this is a major hurdle for new users, so I'd encourage green newbies to start with LyX, especially if you are in a hurry. For PhD students I'd say plunge into using Kile or Texmaker, because if you become overly dependent upon LyX you may get burnt down the road when you need to appy some complex tricks that LyX cannot easily handle, also for large documents it is still quicker i think to write mathematics manually rather than the labourious mouse point and click methods that LyX makes habitual. In that sense LyX is too much like OpenOffice or MSWord equation editor's, what LyX really needs IMHO (to kill Kile and Texmaker and so on) is a MODE switch that instantly converts from wysiwg editing mode to raw LaTeX editing mode, this would be especially useful when typsetting a lot of mathematics. Id be mainly using LaTeX edit mode, and switching to wysiwyg mode when doing things like figure/image inserts. Now that'd be cool! I'd even consider ditching Kile for that ability! But wait there a minute...isn't the "virtual" wysiwyg viewing ability of Kile that I outlined above just the same thing as a MODE switch for LyX? Well, not quite, I still must save and run latex to get a view update in Kile. Maybe in the near future Kile will have wysiwyg modes for image insertion rapid feedback as well! Who knows? It's QuickBuild button is almost wysiwyg---at least on a fast machine---but it's not a true wysiwyg mode.
I wouldn't mind betting however that live update views will become featured with Kile if the developers stay active. Heck,

TeXMacs---for (La)TeX with Macsyma. In the "Honourable" category I will mention TeXMacs. I found texmacs a little annoying for pure LaTeX work. It seemed a bit schizophrenic, a little slow, a little bit Jekyl & Hyde, something in between Emacs, Kile and Maxima, or some strange hybrid of the three. But there is one really cool thing about TexMacs that is perhaps the reason it is a bit strange---it runs a sort of WYSIWYG LaTeX editor that has the native ability to embed (in your LaTeX and LIVE!) lots of other high-level scientific programming languages like XMaxima worksheets and it runs the Maxima calculations (that's symbolic algebra and numerics just like Maple and Mathematica for those who don't know!). Hence the name "TeXMacs" (TeX/Macsyma or Maxima) But what's even better is that TeXMacs also hands you an editing interface to Scilab (a Matlab-like tool that's free and opensource, but not freely licensed), plus an editing interface to Gnuplot, plus an editing interface to Octave and Python and other languages What this all means for the user is quite important. Instead of running separate sessions of say your LaTeX editor and X-Maxima or Gnuplot, Dratex, DrGeo, Graphviz, Macaulay, Octave, Python, R, Scheme, or Scilab, you can instead run any of these computational software applications within TeXMacs and have the input and output embedded in your document. This is just so sweet and beautiful for creating quick tutorials and textbook "ShowMe" type example sets for these scientific tools. The ordinary typesetter may not be impressed, but for an educator writing in-house textbooks and tutorial handouts for students this ability with TeXMacs is simply dynamite, a real time saver if used correctly. All these tools are embedded within TeXMacs, but it's also a wysiwyg LaTeX editor in effect. It comes closest to Emacs as a complete editor/calculator/plotter/symbolic-math tool that you are likely to see in the free software community at present, in fact it surpasses emacs in these strictly mathematical and scientific capabilites. One downside to TeXMacs is that it seems to consume a lot of memory, but that's forgiveable given the scientific power it provides---and it might run beautifully on a 64-bit machine, provided all the scientific software it interfaces is also 64-bit---and besides that, when you are done with your live symbolic/numeric computation and graph plotting session in TeXMacs you can just save (or is it an export?) the latex source and then reopen it in your preferred LaTeX-only editor. Sweeeet!
3. Windows(-only?) LaTeX Editors
LEd is not a diode! Windows users should probably go with LEd. The great thing about LEd is that it has a panel that shows you the DVI, alongside the raw LaTeX, so it looks as good as some commercial LaTeX systems that also provide wysiwyg views alongside the raw latex. You can also plugin your own features and use LEd as an editor for other languages with Add-on packages. LEd would be my pick of the Windows tools for LaTeX editing.
Texmaker could also be used on Windows---it's cross-platform, so Mac users could also use Texmaker (but would also have other options I'd imagine, like OzTeX with the Alpha programmer's editor in (La)TeX mode). MacOs-X also has TexShop, but that tool looks a little inferior to Texmaker perhaps. It seems like Mac users have the most choices, now that they are basically Unix compatible.
TeXnicCenter---for all your LaTeX needs. TeXnicCenter also looks like a really nice tool. It's got everything you need. Honestly, for Windows users there is an embarrassment of LaTeX riches. Unfortunately that'll keep a lot of scientific wordsmith's away from GNU/Linux. Too bad. Now because I haven't played with TeXnicCenter or LEd a whole lot a cannot honestly tell you which is the better editor. TeXnicCenter claim they don't provide a wysiwyg view because they tell you that you do not need it. That sounds a bit condescending to me, especially if you're a newbie to the TeX/LaTeX world. So TeXnicCenter had better have a damned good LaTeX syntax error checking daemon! Let's face it, the beauty of LaTeX is that is soooo different and superior to conventional wysiwyg word-processing silliness that a lot of nice science and artistic folks who only know MSWord or WordPerfect are a little intimidated when they approach LaTex typesetting. So why not give them a bit of a wyssiwyg view? It doesn't cost much to give them that comfort. Then if they get hot with LaTeX they can just disable the viewer and bring it up only when required for finishing. For these philosophical reasons I'm going to suggest that LEd is my favourite Windows LaTeX editor. But please, please, please, don't be discouraged from trying TeXnicCenter! In fact if there's one thing to take way from this mini-review it is that you should play with all of these lovely free software tools for at least a few hours each. The true freedom here is choice and deliverance from bondage. With TeXnicCenter, and indeed with all pure LaTeX editors you are always only a couple of key-strokes or menu mouse clicks away from a nice typeset DVI or PS of PDF view of your masterpiece. After a little playtime you might just find yourself falling in love with the little gem of a tool that you initially thought was an ugly duckling.
WinShell---doesn't "sound" like a LaTeX tool, but it is, and it's a very good one! From reviews I have to give this application a high rating. It's feature set does not seem to include a wysiwyg panel or built-in viewer, The post from darckhart below gives a good rave for WinShell. Note that it's strength seems to be that it has a small footprint, so you can carry a full LaTeX editor around on a memory stick! That won't be so hard in future with smaller and smaller large memory devices coming on the market, but you have to give a big plug to any developers who are making quality software with small memory footprint. So go WinShell!
Also, for Windows users I think WinEdt is a fair tool, though it wasn't free software when i last checked it (some years ago). UltraEdit is another shareware editor that's very powerful and laTeX-aware. But why bother with these when LEd or Texmaker can run under Windows? Moreover, if you are intent on spending money to submit yourself into bondage then why not go all the way and marry yourself to ScientificWord. That's a commercial LaTeX IDE that has the dubious advantage of professional support on demand, if you pay the licence fee. I say "dubious" support because while it is comforting and time-saving to solicit help from a professional at the end of a phone or via an email, if you are really in over your head there is not much that a person operating a help desk can do without physically watching you over your sweat-laden shoulders. So in the long run it's probably smarter to grit your teeth, make some coffee or fruit-smoothie, and settle down and read all (well, not literally "all") the free online help that's available. You'll love yourself more for doing it the tough way, and this way you'll eventually learn to solve complex typesetting problems for yourself and you'll be able to educate your children or colleagues. Whatever you do, don't ever think that you're going solo. There's this vast community of LaTeX power-users out here waiting to help you, you just need patience. I digress however---if life just seems too short to do things the hard way by learning LaTeX in-depth then you probably are not reading this!
Hey Windows users! You can also use Emacs and Kile on Windoze! Just run them from Cygwin. You'll have to install the KDE base stuff though. You shouldn't bother of course, just use LEd instead. If you're a complete beginner then try installing CygWin and run LyX from CygWin. Once you get the hang of writing LaTeX and get a decent grasp on the commands that LyX is really using behind the scenes then you should consider migrating to LEd for more power-usage.
4. MacOS Editors
I started LaTeX on a Macintosh, and it was really smooth. I used the free OsTeX distribution and the Alpha editor. I was very happy with this...until I became a Linux devotee. That was many years ago. Apple have moved on since then, although i could still use these tools. The post below by angelixd (thanks

I believe there are other tools and distros of LaTeX for Mac users, so please feel free to post further comments, I'll try to remember to expand this section as further comments arrive."which is the absolute best tex editor for people scared of the command line. It comes bundled with MacTeX"
5. A Parting Word on Typesetting versus Word Processing
These days I think you can typeset mathematics in MSWord or OpenOffice or KWord using LaTeX. But why bother? What's the advantage? Well, I think it beats the old "Equation Editor" toolets, it was so painful to insert equations with those things. However, really caring scientists will still be using LaTeX for a long time to come because the traditional word processor just doesn't produce as beautiful typesetting. This means general typesetting, not just the mathematical equations!
In addition the usual advantages of LaTeX have not been superseded by any software that I've seen, these include: focus on content rather than layout (the bane of most wysiwyg editors); a massive degree of control over typesetting, portability, programmability (LaTeX is perhaps harder to automate than groff, but writing scripts for LaTeX and BibTeX/Makeindex and so forth, can also be fun and rewarding). There are many more advantages besides these I'm sure (please write about your favourites!).
So by all means make use of the LaTeX equation editors. But the next time your pagination or section headings or headers and footers or figure inserts jump around and scream at you, or your 1000+ page masterpiece breaks down from the memory overload, then turn to the LaTeX guru for help---we can rapidly re-type your entire manuscript into neat and fast LaTeX source.
That's all from me for now. I hope this little review helps some of you young people out there sniffing around for some great scientific word-smithing, word-crunching, free software.