Check out this drum machine/sequencer written in javascript using the latest HTML5 spec.

HTML5 looks to unseat flash as the dominant web, audio/motion graphics platform. Cool!

http://www.randomthink.net/labs/html5drums/

Firefox 3.5 or Safari 4.

From Ajaxian, author Brian Arnold states:

I'm also working on something like the ToneMatrix or Tenori-on (Flash and actual devices, respectively) in pure HTML/JS. It works too, but the sounds aren't exactly designed to be great together (it's currently working on a C scale) and so if you're careful, you can get some decent sound but otherwise, it'll hurt your ears.