What are some good on-line co-op games?

What games do you enjoy playing in on-line co-op? From FPS to sports games what have you really enjoyed? I'm currently playing Golf with Friends and enjoying the simple, sometimes relaxing, sometimes frustrating nature of the game.

Parents
  • left 4 dead 2 is a fun game, but tf2 is always a great option. 

Reply
  • left 4 dead 2 is a fun game, but tf2 is always a great option. 

Children
No Data