Doesn't Doom 3 run on Linux? There's also Doom 1, Doom 2, etc ...
There's lot's of things, such as Civilization, Quake, Armagetron AD ... the great thing is, for most of the games (non-commerical) you won't need a massive CD collection! Plus, if you like FPS, then that's great because the majority of Linux games seem to be FPS.
http://www.gwos.org (Lot's and lots of games: commerical, open source, freeware ....)
http://www.happypenguin.org (Been around for forever and a day)
http://linuxgames.barkingfrogs.net (Starting Out)