BZFlag

BZFlag is an open-source, 3D, capture-the-flag game where you control a tank and attempt to capture the flag from an enemy team (there can be more than two teams in a game). You can pick up weapons and power-ups that will aid you in capturing the flag or defending your base against enemies.

The graphics aren't really that great, they look as though they came from a mid-90's DOS game but with a higher resolution. There is a console perpetually in your face that is slightly distracting. The gameplay is somewhat better, provided that you play the game long enough to get the hang of it. Since you're in a tank, your mobility is limited. You die in one shot, so many times you suddenly die without knowing where the shooter was, or you'll see somebody and not be able to maneuver quickly enough in order to shoot them.

Main Page: http://bzflag.org/
Download: SourceForge
Ubuntu: Available through Synaptic.