Lara Croft and the Guardian of Light
This download-only spin-off title was quite a departure from the games old Lara usually finds herself in in. It utilised a top-down perspective, and it featured co-op! Madness!
Fortunately for us, Crystal Dynamic's departure from the traditional Tomb Raider formula resulted in a fantastic multiplayer adventure with a great balance of puzzles, platforming and action. And there's plenty of room for griefing your partner as well, if you're in to that kind of stuff.
We don't know what possessed Crystal Dynamics to make this game, but it works, and the nature in which players are dependant on each others abilities for the success of each level makes for great co-operative gameplay. More please, Crystal.