Magic Playlist - Get the playlist of your dreams based on a song
Gnoosic - Even if you don't know what you are looking for - gnod will find it.
Spotalike - Spotify playlist with similar songs, according to Last.fm
Musicroamer - A visual Pandora for discovering new music
Playlist Machinery - Tools that help you create & organize your Spotify playlists
TasteDive - TasteDive recommends similar music (musicians, bands), movies, TV shows, books, authors and games, based on what you like.