Last.FM used to have an amazingly accurate and helpful music discovery system, and great similar artist recommendations up until roughly 2 years ago where they either limited the amount of similar artists, or put some sort of blanket filter or whitelist for a smaller pool of "approved similar artists".

Last.FM was so ahead of the game it's mind boggling how they just seem to have fizzled out while Grooveshark, and Spotify came in.

Last.FM had a huge user base with great info on individual music tastes, had a functioning streaming service, a whole functioning ecosystem of music fans and musicians that just seemed to have layed dead in the water after it was bought out.