I forgot about this game for a while, but started playing it again today. It's called Spelunky, and it's a sidescrolling cave-em-up with the randomisation and difficulty of a roguelike. You play as Anonymous Treasure Hunter Guy, who descends into an unknown and randomly generated cave system to grab phat loot. The environment works against you, there are traps, and all kinds of monsters. http://www.spelunkyworld.com/original.html
+1 vouch! Funny you should mention Spelunky, Desi - I used to play it as well and had the urge to download it again yesterday. Amazing little game, especially considering it's free. Derek Yu has released some other pretty awesome games that are worth checking out.