Magic Playlist - Get the playlist of your dreams based on a song
Momixa - Pick two songs and get the perfect mix
Spotalike - Spotify playlist with similar songs, according to Last.fm
Music Suggestions Ninja - Enter an artist's name & find similar, lesser known artists
Playlist Machinery - Tools that help you create & organize your Spotify playlists
Boil the Frog - Create a seamless playlist between any two artists