Spelunky

Back when I was a kid, I used to play a game called Rick Dangerous 2 on my Commodore 64. Spelunky takes me right back.

Just like Rick Dangerous, you control an Indiana Jones type armed with a whip and bombs, and head into underground ruins in search of treasure. Unlike Rick Dangerous, the levels in Spelunky are Roguelike -- procedurally generated, so each journey is different, and they're difficult, so death comes quickly.

Played with a joypad, this is a great experience, with each gaming lasting 5-15 minutes (your mileage may vary, I suck at games in general).

Spelunky is being developed using Game Maker by Derek Yu, and it's freely available.

TIGForums (dev posting and downloads), Gamers With Jobs thread