FIEND.fm - "Hot or Not" for music discovery: 2 hrs of tracks in 2 mins
Magic Playlist - Get the playlist of your dreams based on a song
Music Suggestions Ninja - Enter an artist's name & find similar, lesser known artists
Spotalike - Spotify playlist with similar songs, according to Last.fm
RYSE UP - Connecting up-and-coming artists with the world's top talent
Playlist Machinery - Tools that help you create & organize your Spotify playlists