Cap said:
Quake
Unreal Tournament
Counterstrike
Team Fortress
Tribes
At LAST, someone brings up Tribes! Tribes 2 was one of the most amusing games I've played. It's probably one of the few games where team play means you live longer and win. Also, Katabatic and Grav Cycles (if we're talking about Tribes 2).
The only other one I've played is Planetside (an MMO!) and TF2. Which is fairly awesome. When you're doing well, you feel invincible.
The UT series was wonderful. Not a big fan of UT3, mostly because they nerfed my Flak Cannon and boosted the Biorifle, and then made the game Darker and Edgier, so I can barely see where I'm going. But UT2k4, with it's sheer lack of explanation for anything ("Go kill things. Respawn, then do it again") was wonderful. Colourful levels, gobsmacking weapons, and bots that could at times be far too intelligent.
I've not played much of Quake 3 Arena, except against my brother. But it was fun. Railgun to the face... Bliss.
Oh, and CoD4 is great fun, especially when you're in a tiny level and everyone's using shotguns. Or, in my case, when the only weapon you use is the secondary bazooka. At point-blank range. Hearing the screams from across the room made my day.