TeamComposer is a free, collaborative music composition application.
TeamComposer makes the process of writing music into a collaborative, shared experience. You can chat with other composers and work on the same song simultaneously in your browser, without worrying about trading files back and forth or mac-vs-PC issues.
When you've finished a piece of music of any length, you can then post it to TeamComposer.com, Facebook or Twitter.
Not only can you write music collaboratively with others, but TeamComposer also has interactive music lessons to teach you everything you need to know to get started.