I mostly play X-Moto now. It's a 2D motor stunting game. The reason I play it so often is because a level only takes about a minute, so you can always start the game, hit Quick Play and do a few stunts if you're bored. And it's actually very challenging.
The more serious games I play are:
Descent 2 (d2x)
Return to Casdtle Wolfenstein
Wolfenstein: Enemy Territory
Battlestar Galactica: Beyond the Red Line (might become free when released, but the demo isn't)
And in Wine: Half-Life series, Hitman series, C&C3, GTA2, Dungeon Keeper 2, and Scorcher.
Nexuiz is huge.
Enemy Territory on occasion.
TA Spring (I have trouble managing this properly, I wish there were a better linux front end for setting up games, managing mods and maps.)
Am waiting for Danger from the deep, it seems pretty cool.
I give BSG:BTRL and Vega Strike a shot on occasion. Used to be they taxed my graphics to much and weren't that playable, but I have a newer card now.
Lastly, that 3-D tanks game with the different weapons is pretty fun.