Spelunky is a strange sort of game, a roguelike by way of Terraria. In this game, you play a cave explorer equipped with little more than his wits, some supplies, and a handful of high explosives. Your goal is to reach the bottom of the cave as something other than a pile of bones. Monsters, explosions, cave collapses, and possibly other players make the bones scenario a lot more likely. Check out this review of Spelunky. 1UP has the story.
From the looks of things, a few rounds of the multiplayer would make excellent grounds for divorce or homicide. A little like New Super Mario Bros Wii, except those fireballs rolling out of the other players are lethal to you and the baddies alike.