These are my top ten games, off the top of my head, some are new and graphically impressive, some are ancient but still really fun to play. I know some of these are already mentioned, but I figured I'd vouch for them as well.
1. 0 A.D. - Great real time strategy game, somewhat similar to Civilization.
2. Battle For Wesnoth - Another great strategy game, with more of a JRPG Ogre Battle vibe.
3. Super Tux - Fun Tux platformer.
4. Super Tux Cart - Mario Kart clone featuring Tux, Konqi, the GNU, Beastie, etc.
5. Angry Drunken Dwarves - Fun puzzle game.
6. Megaglest - Another strategy game.
7. Freecraft (Edit- Renamed Stratagus) - Free WarCraft/StarCraft clone.
8. FreeCiv - Another Civilization type game.
9. Bos Wars - Another good strategy game.
10. FreeDroidRPG - A fun Diablo like RPG.
Here are even more free games (142 to be exact) - http://www.linuxlinks.com/article/2009020806275229/Top100FreeGames.html http://www.linuxlinks.com/article/20080522164112313/Games-Part2.html