Spelunky is a procedurally generated roguelike platformer in which you play a tiny Indiana Jones style adventurer hopping your way through ancient tunnels. It's also brilliant. At least that's what PC Gamer's Tom Francis tells us (he's played it over 1,700 times). The Xbox fellows recently got a hold of Spelunky HD, a spiffy remake with much nicer graphics. We don't have that on PC, but we do have something else, the whole original version playing in your browser for free.
Spelunky has always been free, but the browser port is the product of Darius Kazemi , who apparently managed it in three hours, using GameMaker Studio to easily convert it to HTML5. He's hosted it on his Tiny Subversions website, so you can go and play it right now.
If you want to know exactly why Tom has played Spelunky over 1,700 times, check out his Spelunky and the City of Gold article. Although he wrote that in 2009, so he'd only played it 1,233 times back then.