Serious Sam Coop (pure madness)
LieroX (a free game sort of like Worms only not turn based)
Counter-Strike
Quake 3
Warcraft III custom maps
Doom 3 Last man standing (a coop mod)
Unreal Tournament 2004 Siege, Alien Swarm, and Killing Floor (mods)
Half-life 2 Hidden Source and Source Forts (mods)
System Shock 2 coop
Soldat (another free one)
Return to Castle Wolfenstein Enemy Territory (free)
Half-Life 1 Natural Selection (mod)
Pretty much any fun multplayer game, really.
EDIT
Oh and I forgot about the Ship, a Half-life 1 and 2 mod. For HL1 it is free but you have to pay for 2 on Steam. Lots of antics...