I'm not easily impressed by this type of software, but this one really did.
Songbird is GPL (free) based on Mozilla code - and integrates your local media libraries, web radio, other web music sites, media blogs, memory stick files, etc., and all pretty seamlessly. Plus it already has add-ons, Firefox stlye, with undoubtedly more to come.
It's a very early release, yet already works well and promises even more to come.
click here There's a nice video explaining some of the basics.
If you're into music I really think you should give it a go, and see if you're as impressed as I am.