All music you want (almost), instant, and very decent quality streaming. At the price of commercial breaks, which usually aren't that disturbing imo. And at a party, just pay the 1$ for a day pass).
http://www.spotify.com/en/ Try it

Listening tip: The Hit Nation - Somewhere over the rainbow.
edit: ok, did my homework now: Spotify is currently available in Sweden, Norway, Finland, the UK, France and Spain. Win for us
