I'm not in the US so turntable.fm isn't an option. By the way, if you haven't heard of turntable.fm, you should check it out so they get more users and are able to expand sooner.
I found <a href="http://di.fm" rel="nofollow">http://di.fm</a> really good but I'm using <a href="http://grooveshark.com" rel="nofollow">http://grooveshark.com</a> more these days at the recommendation of a colleague. Grooveshark has both radio stations and the ability to pick 'n choose the music.