I often champion Twitter for a great source of information and its because of it Ive found:http://www.we7.com

WE7 offers a massive catalogue of streamed music for FREE!  Adverts are short and between tracks and using the site often leads to an advert free day when youve built up your credits.

WE7 also offers users the option to buy the mp3s (very reasonably priced).

All in all an excellent site, and a great way to support artists without infringing someones copyright!

Its worth noting that you dont even need to register (although Id suggest you do)

Get yourself over now! But before you start searching for your favourite tracks, check out an all time classic!!!(?):

http://www.we7.com/#/album/This-Is-Spinal-Tap-Soundtrack-?albumId=214697