What are your favorite multiplayer games?

[Mummelmann]

Left 4 Dead 2
Team Fortress 2
Counterstrike
Civilization IV
Orcs Must Die (I have the sequel but still haven't played it...)
Neverwinter Nights 1 and 2
Worms
Borderlands 1 and 2
Battlefield 3

Probably more but those are the ones I can think of now.