LittleBigPlanet.
The game has literally the best drop in, drop out multiplayer I've ever seen. Seriously, all it takes is a simple press of the PS button on controller 2 or an invite shot to a friend on PSN, and you can have a second (or third, or fourth) player join you at ANY time, no matter WHAT you're doing. You could be in your pod, waiting for a level to load, or finishing up a level, and your friends will appaer right next to you.
After that, it's a whole lot of slapping your buddies around, and other bits of comic mischief that make the game an absolute blast to play.







