If you want to listen to the greatest online radio, with a huge different kind of music, including the best progressive rock channel (old and new, from TriumVirat to Dream Theater), take a look at the AOL – CBS Online Radio player. This is awesome.
My favorites channels: progressive rock, and Rock Instrumentals.
If you want the best results, connect to this url with Google Chrome, and create an application from this url. You'll get a standalone player separated from the rest of the browser.