Last.fm is available free stateside, as well as UK and Germany, elsewhere in Europe it costs €3 per month, not sure about rest of the world. Similar operation to Spotify, sign up, set up your own library, select complete albums or just tracks, select by composer, artist or genre, I have a mix of jazz, baroque classical, big band swing, sometimes add a few prog rock. It's a good idea to set up more than one library, or to change some artists/composers from time to time else you could get a bit fed up with some tracks. Some days when I have it on for 10 or 12 hours the same track will get played more than once, although track selection is fairly random.
http://www.last.fm/