Hell is Other PeopleThe premise is that you're a airplane and you shoot other airplanes. It's 2D, it's really simple. It's also multiplayer.
But not really. See, what it does is when you play, it records what you do, and then uses the 'ghost' of your ship as the trajectory of one of the opponents for the next player that plays. It starts relatively easy, but slowly adds more ships, so not a single game is the same, and as people realize new tactics the game develops into something completely different. It's concept is pretty amazing.