Previously mentioned Spelunky has updated to version .99. Improvements include bugfixes, a new secret or two, and my personal favorites, some interface enhancements. Joypads work properly now, a 4x resolution muliplier for higher resolutions to retain crisp pixelly graphics, and adjustable sound. I no longer have to change resolution, run Joy2Key, and drop my sound down to 1% to play.
Do yourself a favor, and download this wonderful game right now. If you can, don’t read too much about it, since general gushings about this game will often spoil a few little surprises about it that would be much more fun to discover on your own. Also, once you get tired of the looping built-in music, I recommend you tune it down to nothing and play some good stuff from the OCRemix project. (There are worse places to start than here.)