Good on line multiplayer games

Duck Game just came out and its getting praise.

Well if you're interested in the 4 players racing style, I suggest Giana Sisters: Twisted Dreams although I never tried it.

EDIT: seems like you already own it. May I ask for some feedback?

Battleblock Theater or Castle Crashers are great games - semi co-op, semi-make your partner annoyed :)
Magika is a fantastic game if you can forgive it's still inredibly buggy state (1-2$ is fantastic price to it's experience and it's fun) It's a co-op, but you can accidentally (wink-wink) kill your teammates. Repeatedly.

You might want to try Kill Fun Yeah. It even has a 1-hour free trial. I never got to play it more than that because the game asked for the steam key to make my account non-trial, and indiegala didn't provide that at the time. Oh, and also Plain Sight, but sadly that game is as dead as jesus.

Pool Nation, The Ship, Mount and Blade. But you better search for the on steam

the list of the games you search