Grooveshark
As I said previously, Grooveshark, is a free, legal, online streaming music site that allows anyone accepted to listen to music online with virtually every song ever created. All you need to do to get in is sign up at the registration page. You could also email me here and get an invite promptly in which case we all are positive.
Grooveshark does not use standard HTML pages, in which one would need to load the pages over and over again to lose the song that they are currently listening to, but use certain javascript applets to load the song as you transition pages. It is generally a great site and would have to rate it a 4.5.