After work today a co-worker and I played a few rounds of OpenArena, a new playable game derived from Quake3 that is fully distributable under the GPL.  The company that produced the original game Quake3, ID software, released the game engine but not the rights to the graphics.  So the fine folks at OpenArena combined the donated Quake3 engine with their own graphics… and the combination makes for a very enjoyable game!

They have a game client for both Linux and Windows, and seems like maybe something for OSX too.