Well the "big" guns in the Open Source FPS crowd are:
Nexuiz
Warsow
Tremulous
Sauerbraten
Cube
Alien Arena
I'd have to say Nexuiz and Alien Arena are the most fun. Nexuiz is like Quake 3 on cocaine, it's all about reaction time and lightening quick reactions. It's kinda laggy though.
Warsow is fun, I guess, but I never really liked it.
Tremulous is fun for a week or so, but it really gets old terribly fast.
Sauerbraten is sort of a weak attempt to make a new original Quake. Cube is what it's (originally) based off of.
I'd recommend downloading Quake 1 (not technically legal, but not as morally painful compared to say, downloading Bioshock) and installing Darkplaces. It's not a deep and mystical gaming experience, but it'll knock some hours out of your day.