Yes, I know this is totally old news from like 3 days ago, but I have to post it. Songbird is a web player built from Firefox’s browser engine. Songbird is open source, will run on Windows, Mac OS X and Linux and supports user contributed, cross-platform extensions. It also intergrates with not one, but all of the online music stores. It’s 0.1 at the moment, it has along way to go, but looks promising. Link