Songbird (http://www.songbirdnest.com/) is an example of how the firefox engine can be mashed up with some additional functionality and a website to create an excellent tool.

Still in the early stages of development, once this gets all it's bugs ironed out (and provides some method of syncing with portable media players) I think it could become and important player in the online media market, providing the indie labels with a workable alternative for distributing their own music.