First of all, Quake 3 = eww
Otherwise, there are these two mods for UT2k4 called Troopers 2004 which is a Rebels vs Stromtroopers Star Wars game, and Air Buccaneers, which is the most fun you can have in a balloon with a cannon in it...seriously!
For HL2, there is a single player mod called Minerva that just released its first chapter, it's pretty good!
However, i think all of these mods rate as pretty big downloads, so i don't know what you can do about that.