Songbird is a Web player built from Firefox’s browser engine. It’s is open source, will run on Windows, Mac OS X and Linux and supports user contributed, cross-platform extensions.
Ars Technica have some great coverage, as ever. It’ll be interesting to see how it turns out, think they’re trying to pitch it as a competitor to the iTunes monopoly. I agree with what Ars say about the difficulties of multiple online stores, though perhaps a well-written plugin for stores to share a common payment gateway such as PayPal may overcome this, or, as Ars suggest, someone may step up and create a new one. At any rate, it’s worth keeping an eye on Songbird, she might well grow up and sing?