Spotify is a hugely popular music subscription service launched out of Sweden in 2008 that boasts one of the largest libraries in the streaming space. Part of its appeal is its more global approach than some of its competitors, and that Spotify is available truly cross platform. Mac or Windows, Android, iOS or Windows Phone, Spotify is available to them all.
In Europe, Spotify is pretty much the King of the hill. While the United States had alternatives such as Pandora and Rdio, Europe had Spotify. As such its well established in that part of the world, while still struggling to make serious in-roads in North America.
If you can think of an artist or album, there's a good shot Spotify can serve something up for you. Then you can also create a radio station based on it to help discover great new music. Want the music but don't want to pay? Not a problem, as Spotify now offers mobile access to its streams through its Android and iOS applications so long as you don't mind things being shuffled.
A Premium subscription at £9.99/€9.99/$9.99 a month gives you offline access to every single one of its 20 million odd songs in 55 different countries around the world.