Check out http://vexflow.com It's got a lot of interesting features and the code is really usable... it has tab support for the guitarists (like me) who can't read music, it's open source, html/svg rendering, and it's got on a pretty interesting text based DSL for authoring music. I even wrote a plugin to integrate VexTab on Mediawiki but I haven't done much with it after the proof of concept. Too busy!