BandNext - Discover bands that sound similar to artists you already love with BandNext. A single click saves your results to a Youtube playlist.
Magic Playlist - Get the playlist of your dreams based on a song
Spotify - Map shows when two people play same song at same time
Spotalike - Spotify playlist with similar songs, according to Last.fm
stats.fm - With the click of a button you'll be logged with your Spotify account and you'll instantly gain access to a valhalla of cool stats and insights.
Playlist Machinery - Tools that help you create & organize your Spotify playlists