I have the following...

Rocket Arena 3 for Quake 3 <---AWESOME mod, built in mp3 player, a DEFINATE way to polish up those skills, but be prepared to get schooled.

Excessive for Quake 3 <--- just balls to the wall explosions with alot of sailing through the air, just an all out fun mod

Weapons Factory for Quake 3 <--- have gotten around to trying yet

Quake 3 Fortress <--- good tema based mod as long as you are playing with some good people

Tactical Ops for Unreal Tournament <--- play occasionally, similar to Counter Strike but better

This is one I no longer have installed but was pretty dang fun, Rocket Crowbar for Half Life <---a funny mod where you shoot all kinds of wack stuff from the guns.... rocket launcher shot out scientists that exploded if you got to close