I learned about Google Music last week from Diep's Jeff, and its pretty cool as far as I know - you can upload your whole music library to a server and then stream it through ur google account anywhere - ur iPad, Android phone, iphone, any computer, etc... Amazon also has the same thing, I think there's a deal right now to get an unlimited space acct for like a buck or something
But then I came across this http://www.spotify.com/int/coming-to-the-us/
It quotes Mark Zuckerberg: "Spotify is so good." Not exactly elaborate, but wth I believe him. Apparently, its like having every possible song in the world.