Nexuiz

Alright, Nexuiz is a FPS based off a HIGHLY modded version of the quake 1 source (darkplaces engine). This fork of darkplaces is a full standalone game with an extremely good lighting and graphics engine overhaul, ragdoll physics, the works. It is overall a very nice and pretty game to play in general. In ubuntu you can simply install this with "sudo apt-get install nexuiz", and that will get you going, and put the game on your menu. Alot like quake 3, the only single player is playing against bots, but if you like that sort of thing, you will like this.