I absolutely love procedurally generated content, and it still surprises me how the vast majority of gaming seems to ignore it. I've played countless hours of Nethack, was absolutely enthralled by the first Jade Cocoon, and now I'm steadily losing my life to Spelunky. (Thanks to Darius for singing its praises.)
Imagine, if you will, an Indiana Jones-inspired platformer, forced onto the SNES. It's a dungeon-diving game, with a progression of floors, changing as you get deeper and deeper. For instance, you start off in a cave-temple - but at level 5, a curious overgrowth of forest appears. Monkeys and all.
Now, imagine that every time you play the game, the levels change. You'll never play the same thing twice. One playthrough, you may find yourself starved for bombs and grappling ropes by the second level. Another, you'll be merrily sending a huge wall-breaking boulder tearing through one of the anomalous subterranean shops.
You'll get to see a lot of different levels, because you'll be dying quite a bit. It's a brutal game - though much more forgiving than Nethack, that's for sure. Healing is often, but taking damage is even moreso. Even if you do manage to stockpile a lot of health, though, there's plenty of one-hit-kills to keep the game dangerous.
I'm going to put this up there with Dino Run, for quality retro-graphic games. Now if you'll excuse me, I have to go play it again. Need my fix, and all.