UFO: Alien Invasion
http://ufo.myexp.de/
I'm not sure if any of you remember a game from back in the days called X-com: UFO Defense (North America) or X-com: Enemy Unknown (rest of the world) but in many old gamers opinion it's one of the best games ever made. UFO: Alien Invasion is a homage to this game developed by the open source community with vast improvements in the graphical department (modified quake 2 engine), gameplay, sound, and overall feel to the game. I am thoroughly addicted to this game.
Space Invaders OpenGL
http://spaceinvadersgl.sourceforge.net/
Yeah everyone and their grandma has heard of space invaders, but everything is better in openGL! It's still the same old Space Invaders, even some levels look like the cheesy Space Invaders of times forgotten, but that's blown away as the levels scale rotate and do other strange things. It's a fun way to kill time, that will never get old. It's no gaming experience, just mindless action. Kinda like a one night stand.
Chromium B.S.U.
http://www.reptilelabour.com/software/chromium/
It's fun yeah, it will frustrate the HELL out of you. Most fun top down shooter I've played since Ikagura. Is this game on Ikagura's level? Hell no! Is it fun to play and will it frustrate you? Yup. Most of the best free software games are these little time killers, but the damn thing is addictive. If anyone actually get past the 3rd level please post me a screenshot so I can look on in evny.
Frozen Bubble
http://www.frozen-bubble.org/
Anyone who has ever used Linux knows about Frozen Bubble. For the longest time it was the only non shitty free software game out there. It's premise is simple. Shoot little colored balls from your cannon into the other like colored balls and make them go away. Sounds cheesy but again, you will play to kill 15-20 minutes, and find yourself an hour later checking the time like “Oh shit.” It's on almost every platform out there, from OSX to Windows to Linux to BSD.
Battle For Wesnoth
http://www.wesnoth.org/
Holy crap, a free software game on par with commercial games? Impossible! Not really, while now the graphics look dated, but when initially released the graphics were superior to Warcraft 2. This doesn't seem like much now, but Wesnoth is a really engaging game. I've been playing this thing for a few years now, and one thing I like about it is an abundance of additional campaigns. If you like turn based strategy games, you'll love this thing.
Glest
http://www.glest.org/en/index.html
While still in heavy development, this is shaping up to be an awesome real time strategy game. It's very playable at this stage, and quite enjoyable. It's a great looking, playing, and feeling game with a unique approach to differentiating armies from one another. You have magic based armies and tech based armies, both with their respective strengths and weaknesses. Their respective tech trees are also very balanced and varied.
Spice Trade
http://www.spicetrade.org/index.html
Um this game will take some work describing. It's a little weird, yet kind of engaging. It's by no means the best game I've played, but it's consistently fun, and the idea is something I've never come across before. You are an arabian spice trader and you go about the world trading spice, etc etc. The game again sounds strange and it is. But it's good clean fun, especially when you get a genie to handle your accounts.
Scorched 3D
http://www.scorched3d.co.uk/
A remake of an old MSDOS goodie called scorched earth, this one takes the principle behind the gameplay and ups the ante to a 3D artillery style game. All of the environments are destructible, and there is nothing like playing online against someone. The sheer evil you can inflict on other players is juicy and spankworthy. Spankworthy enough to where if enough people wanted to, we could have a mindwerks scorched 3d tournament. And the Death's Head looks beautiful FTW!
LinCityNG
http://lincity-ng.berlios.de/wiki/index.php/Main_Page
Well if you can't tell from the name, this is a SimCity knockoff that is actually just as good. It's based loosely on SimCity3000 and has great looking graphics and it really is a blast to play easily rivaling the original. It's SimCity basically, what more can I say?
FreeCiv
http://www.freeciv.org/index.php/Freeciv
More homage to oldies but goodies. I'm not sure how many of you remember Civilization or Civilization 2, but here is a free software version with beefed up graphics, enhanced ai, online play, and a variety of user mods. If you don't know about Civilization, then you just need to try it out. For those of you who do, yes it's as good if not better.
Crack Attack
http://aluminumangel.org/attack/
What would a free software/open source games thread be without a tetris clone? Well in this case I picked my favorite out of the thousands (no bullshit, at least thousands) that are out there. It's roughly based on Tetris Attack which was a fun twist of the orginal while still retaining the charm. Not much else to say, it's pretty tetris.
Boson
http://boson.eu.org/
A real time strategy designed to feel like Warcraft and Command & Conquer, it's a shame this game hasn't been ported to windows yet, as it is gorgeous to see, and fun as hell to play. It's open source which means it can be ported to windows and as soon as some one takes up the task it will be, in the meantime I almost feel guilty placing this one on here being as so many people only have Windows, but this game kicks so much ass it's justified. Just RTS bliss. Just bliss.
Free Orion
http://www.freeorion.org/index.php/Main_Page
Another tribute to greatness, Free Orion is an open source implementation of Masters Of Orion released in the mid 90s. A turn based space exploration and planet conquering strategy game, Free Orion has vastly improved on the game to which is pays tribute. You can be any number of races each with it's own strengths and weaknesses. Really this is just another one you have to play before you really know.
This next game isn't open sourced that I know of but so damned good I had to include it.
DUO
http://www.binaryzoo.com/games/index.htm
I'll describe it like this, you are a little balloon firing shots off at other balloons. Sounds simple and it is, at first. The graphics are simple yet gorgeous. And the gameplay, well it's one of the most unique shooters I've ever played. You just have to try it. Seriously, it's that fun. As well as it's cousins DUOtris, and MONO.
