Virgin Radio on your phone!
This rocks! It works very well on my 6600 over my unlimited T-Mobile US GPRS internet access point. The sound quality is quite good -- Led Zeppelin's Dazed and Confused came through loud and clear with no distortion. My only criticism: it would have been nice to display the band and title of the song being played on your phone.
One other thing I noticed: it seems like it puts some stress on the processor. When you are using another app, the music breaks up a bit. I listened quite a bit today, and it ran down the battery a little faster but not drastically.