Spelunky is a very polished platformer with procedurally-generated levels and great replay value.
Everytime you play it you will notice that the game is a pure labour of love: Lovely pixel art, smooth controls, well balanced gameplay mechanics and fantastic music… It’s an awesome masterpiece that clearly shows that platformers will never die!
The original PC version is available for free and the new Spelunky for XBLA (with HD graphics and 4 player mode etc.) is available for 15$. I think that the new version will be released for PC later this year.