I'm probably the last one to know about this (and when I read about this morning, I remember someone showing it to me earlier), but if you like online music radio, you should consider Pandora. It is a free service that builds stations built around your tastes.
What does that mean? I created my first station by entering Public Enemy. Pandora recognized this as old school rap and began to play music in the same theme. I then tried the Charlatans UK and Depeche Mode. In both cases Pandora was pretty dang good, almost scarily good, at finding music that was similar in nature. At the same time, it is playing a lot of good music from bands I've never heard of.
For each track, you can ask Pandora to tell you why it was recommended, as well as how to purchase the track.
I did run into two small problems. The first is that the Flash-based music player would stop every now and then when Firefox loaded another page in other tabs or windows. I just opened up Pandora in the Devil's Browser and that solved that. The next problem is that it seemingly had no idea who Lush was. Maybe it just doesn't get the shoegazer genre, but I'm going to try a few alternatives to see if it can figure it out.
Have I said before how much I hate corporate radio? Or to be fair, over the air radio. XM and Sirius seem pretty cool. Maybe if - lord forbid - radio stations actually played music ... no - I'm going to stop now. This will not be another rant.