I’ve discovered a very cool online radio app: Last.fm Radio Widgets. You can choose your own music styles using “tags” and stream great music all day long. Widgets exist to place in blogs, Facebook/MySpace pages, etc. You can also navigate to the page of your choice (using genre tags, artists, etc.) and simply listen to the station as a tiny pop-up window. Very well done and great music choices.
Here is an example of their Progressive Rock widget. Click to start: