FIEND.fm - "Hot or Not" for music discovery: 2 hrs of tracks in 2 mins
BandNext - Discover bands that sound similar to artists you already love with BandNext. A single click saves your results to a Youtube playlist.
Music Suggestions Ninja - Enter an artist's name & find similar, lesser known artists
Gnoosic - Even if you don't know what you are looking for - gnod will find it.
RYSE UP - Connecting up-and-coming artists with the world's top talent
TasteDive - TasteDive recommends similar music (musicians, bands), movies, TV shows, books, authors and games, based on what you like.