From the Songbird site:

Songbird is a desktop Web player, a digital jukebox and Web browser mash-up. Like Winamp, it supports extensions and skins feathers. Like Firefox, it is built from Mozilla, cross-platform and open source.

I've heard that Songbird was developed by the original developers of WinAmp. So far it looks pretty cool and the fact that it's a XUL app is quite awesome.