For the geeks (if they haven't already heard of it, that is), Songbird is built on Mozilla's XULRunner interface, which allows seamless integration with all platforms, be it Windows, Mac or Linux. For the not so geeky, Songbird is a player that, apart from serving as your everyday desktop based media player, plays media from the web. You can browse web sites from the player itself and if you land on sites that contain music it will crawl that page and create playlists from all the media present on that page. What more? You can just click on the song in the playlist and it will play it just like it will play your local audio. It downloads the ID3 tag information and everything, and you can save the song by dragging and dropping it into your library. Yes it's as simple as that.
The screencast at the end of the post will better explain what this player is capable of.