The best part is that when someone codes and posts an app there, it becomes community property, so anyone can branch off their own version and change it at their whim. You can view and download the Actionscript code.
Most of what you'll find are little graphic "demos" that show off the coder's imagination and skilIs. It's reminiscent of the old Demo Scene... a social gathering of creative developers and artists sharing and competing in a fun atmosphere. I don't want to date myself too much here, but I (only sort of) miss the heydays of the Demo Scene where hacker/artists would code up the most amazing little works of art into tiny little files. I spent many an hour running old demos on my Amiga and Atari ST... wondering how on earth they pulled it off on such simple hardware.
... but I digress.
I haven't had time to dig around the contents of wonderfl much (Heh heh... free time, what is it, exactly?), but I think that this one is pretty cool, interactive blobs. Try moving your mouse around and clicking.... Very satisfying.
Anyways, there's lots of fun to be had, if you click around a little. I unfortunately have no time for fun right now. ;)