One of the highlights of my recent trip to GDC in San Francisco was hearing Jason Rohrer talk about his game Inside A Star Filled Sky - a game that plays with concepts of infinity, disguised cunningly as a traditional Shoot 'Em Up. The game is now available on Steam from the reasonable price of US$8.
Inside A Star Filled Sky is genius. Die, and you withdraw into yourself. You yourself transform into a level. Not only that, but you can enter into the bodies of difficult enemies, who themselves become levels, and transform them from the inside, hence making them easier to defeat.
The game soon becomes a maelstrom of ideas and leads - you leap inside an enemy to change him, die inside the enemy, withdraw into yourself, and before you know it the tasks you set out to achieve have been completely transformed. You become lost in layers within layers of different meaning.
Then your head explodes.
Give it a try!