Two words (combined into one word): TuneGlue. 

This new online “music map” serves as one of the most complete music suggestion sites on the web. It seems that places like www.pandora.com have opened the floodgates for music recommendation. TuneGlue is a powerhouse of RIYL information, combining knowledge from both Amazon’s and LastFM’s relationship data. And it’s oh so easy to use! 1. Think of an artist you like. 2. Type that artist in TuneGlue’s search bar. 3. Watch as a little “pod” appears. 4. Click on the “pod” and hit expand. 5. Watch as a flurry of similar artists erupt in all directions from your original search. 6. Lather, rinse, and repeat. Enjoy!    

BTW, here’s the address: http://clients.onyro.com/tuneglue/