As you should be able to tell from my gamercard on the right, the game I've been playing lately is Geometry Wars. It is totally excellent. Gameplay-wise, it is like Smash TV or Robotron, if you know what either of those are.
Graphics-wise, it is the most psychedelic thing ever created. There's really no way to describe it, it has to be seen and played to be understood.
It's a good example of what the XBOX Live service can do. If you couldn't get the 360 on the internet, there'd be no reasonable way to distribute this game. After all, no one would pay 20$ for it, but it is a legitimate game in its own right.