http://www.audiotool.com/

This is deeply impressive, and the first decent thing I've seen Flash do in months. Try doing this with HTML5.

It's a music composer using "devices" like Propellerhead Reason, but not constrained to a rack, which I think is really nice. The sequencer is a bit tricky to start with, but everything else is really easy to use.

Once you've created your track, you can tag it, publish it, share it, etc etc.

I managed to knock together a simple (if not exactly exciting!) loop in 5 minutes:

http://www.audiotool.com/track/funkybeat/0

Let's hear your tunes then

--
Matt Doyle, Elated
3rd Edition of my jQuery Mobile book out now! Learn to build mobile web apps. Free sample chapter: http://store.elated.com/