My older brother informed me about Grooveshark weeks ago.  I finally was able to try it out yesterday.  This type of internet radio blows Last.fm and Pandora away.  Just being able to customize your playlist and being able to repeat the song over and over is just GREAT!  With Last.fm, you just input your favorite band/group and the system chooses the song.  You cannot replay, just fast forward.  The same works for Pandora.

I think you should try Grooveshark and enjoy its unique service.