The Battle for Wesnoth is a turn-based strategy with very simple gameplay mechanics that somehow lead to a tremendous amount of depth.  Anyone can grasp the simple point-and-click aspects of it, so it's easy to pick up and starting playing without going through an exhaustive manual, but the interactions between the various units, their abilities, and the environment all allow for tons of strategy.  It comes packaged with several campaigns and user-made content can be downloaded right from the game interface.  While some of the campaigns it comes with are somewhat lackluster, one in particular (Under the Burning Suns) is the greatest gaming experience I've ever had, and one of the better storylines I've read/seen/played.

Glest is a real-time strategy game with 3D graphics.  While the basic (now abandoned) Glest engine/game lacks a lot of features that one would come to expect from a modern RTS, various projects involving the continuously-developed Glest Advanced Engine aim to fix that.  GAE itself supports just about anything you'd expect from a modern *arcraft-style game, but sadly the actual available content is a bit slow in catching up to the engine's features.  The biggest lack, in my opinion, is of a campaign/story mode.  There is a very active modding community with tons of user-generated content.  There also exists an alternate fork called Megaglest, but... meh.

Warzone 2100 is another RTS I enjoy, but quite a bit different from Glest.  One of the most impressive features is the modularity of the game's fighting units.  The player can build new vehicles by mixing and matching different chassis types, propulsion (wheels, tracks, hover, etc.), and dozens of types of weaponry.  I'm about 1/3 done with the campaign, and it's quite solid so far.  I do get occasional crashes, though, and the interface takes a little getting-used-to, but I quite enjoy it.

All three are under the GPL and available for (at least) Windows and Linux. Glest and Wesnoth are also available on Mac OS X, but I'm not sure about Warzone.  GAE should compile on a Mac, but I'm not making any promises.

I'd like to confirm that Warzone 2100 is available for OS X. :)

Wesnoth and Summoning Wars both rock. Glest is awsome too, but from a moddabiloty point we 2D/non 3D artists suffer somewhat due to our lack of modelling skills. 3D modelling is something I'm still figuring out...

ahahaha, yes, me too, well.... my modeling skills are zero. :(

Well i'm playing the free account of ryzom morpg... simply... wow! (not w.o.w)... WOOOOOOOOW!
When you enter in the world... you feel like an avatar! (yes, from the movie)

Another 3d morpg is http://www.evidyon.es/, well....  the site is awesome, the game need more work, in pro: the server don't have lag, The proyect is in constanly development and the developers are very cools, wich means a lot. 

uhm... welll i guess everybody knows Hedgewars, my girlfriend love it... i hate it (she allways win -.-)

I forgot to mention one game that i played many times... Warlock =)

Link to warlock site

I like playing super tux kart from time to time, it's gotten pretty good by now.

I think the most promising open-source game is 0 A.D., and realistic RTS that is being developed by Wildfire Games.

Yeah, 0 A.D. does look promising.  Their water and terrain are especially awesome.

Why do half of the FOSS type games I've heard of seem to be RTSes?  It's a little frustrating for me, not being a big fan of the genre...

It might have to do with how demanding the art content is.  Making a decent-looking RTS character doesn't take as much work or skill as making a high-poly FPS character, for example.  There are no shortage of FPS, however.  Warsow, Alien Arena, Nexuiz, Open Arena.  There's a really good looking one in development, called Overdose, but it looks like that one is a long way off and I don't know if they're freely licensing their assets or not.  I'm pretty sure that all of the above use various Doom/Quake engines.  Cube is an open-source engine, but the art assets of the various games/mods/whatever are under various different terms.

Johndh: Are they even opening their code? Maybe they licensed the quake engine they're using, as some -silly- people do

Redshrike: For me, being a fan of historical RTS games, it seems like there's an overdose of FPS and isometric RPG's. :P

Actually, I don't think there are that many FOSS RTS games, unless you throw TBS games under the same category. We have Glest with continuations, which I've never cared to play, we have Warzone 2100, FreeOrion, Widelands and we have 0 A.D. I can't remember any other RTS games worth mentioning, and out of these five, only 0 A.D. are truly historical and three-d. So I think it fills an important niche since a few years ago, historical RTS games were really big in the commercial world, and there are possibly huge user bases for such games out there.

@TheAncientGoat: Warsow's engine is GPL but the content is closed.  Open Arena's engine is GPL, but I couldn't find any information on the art content.  Same goes for classic Nexuiz and Alien Arena.  It turns out Nexuiz is getting forked into a new project called Xonotic because the developers want to use the Nexuiz name for a closed-source console game based on the Crysis engine.  According to some folks on IRC, Xonotic is entirely GPL, including assets.  Cube uses an entirely different engine, which is licensed under ZLIB, but the art assets are as I said.

@johnd You covered each project except the one I was talking about, hehe... Overdose. I know the status of all the other projects, I blog for freegamer, guess I should have been more specific

Open Arena and Nexuiz/Xonotic are strickly GPL (including media). AlienArena's content is un-free.

That's because I don't know about Overdose. :)  I'll do a little digging and see what I can come up with.

Edit: "The full OverDose source code is released under the terms of the GNU General Public License." [source]

I still couldn't find anything about the content.  You could always contact the team if you're dying to know.

Yeah... but actually the souce has not been released yet AFAIK (contrary to what it says in their wiki), they are still cleaning it up since 1 year or so. Probably will only happen when they artually release anything. Media is for sure closed... they also though about buying a license for the engine from id, just to keep it closed too... so I wouldn't be surprised if the code never gets released.

Otherwise really nice project though.

@arthur, there is also Spring Engine and it's Games. It has everything from Star Wars, Total Annilahation Mods, Spring:1944 a WWII RTS, and many more. All three-D and most fun.

@verbalshadow: Yeah, Spring does seem interesting, but I didn't really understand the concept of how it works. I tried installing the Spring client once, but was greeted with a message telling me I had too old a version... and the games that run atop of it seems to be mostly multiplayer only? I may be wrong; I have to be honest and say I didn't really research it, but IMHO a game shouldn't really require much research.

I've played allmost all the games mentioned so far, I'd say Wesnoth can be loads of fun, however it can be rather counter intuitive at times, and modding it is a pita, after working on a new race for it I got totally turned off pixel art. I think it would work really well on a touchscreen though and I've been waiting for it to get ported to android, I know I would play it constantly if it ever makes it across. From a user perspective I'd say it needs more unit and environment animations, but having tried my hand at making some, I can definately see why its lacking in that area.

 Glest has potential, but it feels quite ponderous and clunky, I suspect some of it is UI, but some of it seems to be design choices around build order, build times and responsiveness.  Still, I hope to see more progress on it as time goes by.

 Warsow/Nexuiz/Alien Arena/Tremulous/Saurbrauten all seem to be the same game to me, unreal tornament. Its like the FPS genre moved on to greater realism and intergration of rpg elements but all the FOSS shooters stuck with the old hyperfast twitchy-jumpy multicoloured fragfests. While the engines are an awesome achievement, it would take some serious changes to gameplay for me to be won over by any of these shooters. I like the look of CubeCreate, but having seen what that community did with Javascript, sadly I can't imagine the lua version will amount to much more than a series of casual minigames. 

  On the rpg front, there seem to be a billion and one 2D or isometric games, however, while I can sometimes get into a retro mood, for real immersion 2D just doesn't do it for me anymore and I can never manage more than an hour or two on any of these. I did my time on nethack clones, and I know it seems shallow to say this, but until one of them has a decent UI and some 3D graphics, I've had my fill of that genre too.

  There are of course the mmorpgs, the likes of ryzhome and planeshift, however they all seem to be stuck in a timewarp, clinging to horrid gameplay mechanics and aging graphics like a badge of honour. Again I hate to be a graphics whore, but the textures need updating and the animations are jagged and weak, and in the case of Planeshift in particular the world seems abandoned, empty and lifeless. I wanted so much to like planeshift, heavily roleplay oriented and a unique world idea, but instead I found myself alone in a blocky world hunting rats with a blunt knife and no idea why. Any game that does that to me gets dumped.

 I'm quite liking UFO:AI, which is why I've been doing some texture work for them, but it still needs a lot of work before I would seriously recomend it. I really hope to see a necromunda mod come out of it someday, however, once the storyline mode is complete and a few bugs get ironed out, I think it will be a really solid game, even if it does suck resources like nobodies business on Linux.

 I am really looking forward to the Arx Fatalis port since it was open sourced, and freespace has made wonderful progress since being open sourced too, so has 0a.d but it is a bit sad that the best FOSS games around seem to be all ports of ex commercial games.

Personally I am still waiting for a FOSS version of something like morrowind or fallout 3, however It seems like I'll be waiting forever since no one seems to be working on anything like that.