Heroes of Newerth is a moba game similar in vain to dota.
Savage2 is a fps/rts team based multiplayer game pitting humans against beasts. Based around melee combat, you'll witness some epic battles. Have you got the pebbles to stand up to the hellbourne?
Tremulous is a standalone game based on the Quake 3 engine designed specifically for team based mulitplayer gaming. It is freeware & open source. Give it a spin. Kill some Bugs, Eat some Humans. Have some fun.