Spotify is a music platform based out of sweden. It originally started in 2006 and last year became available in the United States.
The catalog of music is huge. Just search for anything and you're likely to see it come up and listen to it. You can create playlists and stream them whenever you want.
It's linked to facebook so you can see what your friends are listening to either when it posts it on Facebook or right on spotify itself.
It's available through your mobile device as well as your PC or Mac.
There are different levels. if you want to pay for Premium you can stream an umlimited amount of music with no advertisements as well as have it on your phone. I just have the free version that I can stream millions of tracks instantly and you can subscribe to others playlsts and hear their music. http://www.spotify.com/us/