Songbird is an open-source media player based on Firefox, so it's also a browser. The software lets you play music from the web as easy as if it was on your computer. If you go to a site that has links to music, Songbird creates a list of of files you can download or play.
Songbird gives an easy access to radio stations, podcast services like Odeo, network services like last.fm and Creative Commons music.
Songbird can play many music formats including MPEG Audio (mpga), MPEG Layer 3 (mp3), MP4 Audio (mp4a), Ogg Vorbis, Speex, AAC, WMA, FLAC.
The program is currently at version 0.1 and will grow much more when developers will create cross-platform extensions and plug-ins.