This Flash-based widget from Box.net allows you to share all kinds of files: anything on your computer, plus music, photos and videos. Many of these file types can be viewed in the widget itself: you can view a photo, for instance, or play an MP3. You can sign up within the embed itself, and upload files to MySpace, Facebook and blogs in general.
The Box.net is cool but for music try http://www.sonific.com – 20 secs to get a cool and perfect streaming music widget. Fully licensed, too