I know it might be hyped by now but: DayZ
It is a zombie apocalypse mod for ArmA II: Combined Operations which let's you join servers, create a charachter and then explore and scavenge, kill zombies and possibly other players (That is up to you). However when you log out on one server and log into the other you will be in the same spot as you were before, and when your charachter dies you lose all your stuff like in minecraft although you probably won't get it back in this game. In that sense it's actually more of an MMO.
Anyone playing this? Wanna team up?