Battlefield Bad Company 2 <<< This is my favorite game right now, I cannot wait for BF3 to release in a few months!!!
Little Big Planet (online and split screen)
Borderlands (Although it is sometimes difficult to find people to play with) (online and split screen)
Battlefield 1943 is a fun game too.
There are a lot. Battlefield 1943, Battlefield 3, Uncharted 3, MAG, Burnout: Paradise, James Bond: Blood Stone, and Warhawk is all I can think of right now.