ChordFork is a simple song hosting/cvcs application which I have begun working on for the class CS184. Users will have the ability to upload songs, where a song is currently a simple chord progression, rework those songs over time while saving different revisions, fork other users' public songs, and share songs with other users or friends.
ChordFork is currently very alpha and can be tested at chordfork.com. It is built on node.js with Express. I am running ChordFork on Heroku with the PostgreSQL add-on and using sequelize to interface with the database, although ChordFork can also be run locally with a local Postgres instance.
In the long term I would love to make ChordFork into a full-fledged score editor in the browser, but as of right now solving the UI/UX problems associated with writing scores in the browser and engraving sheet music in real time are both outside of my expertise and too time-consuming to tackle.