The Wub Machine - Turn any music into Dubstep, Drum & Bass, and more.
Magic Playlist - Get the playlist of your dreams based on a song
The Eternal Jukebox - For when your favorite song just isn't long enough
Spotalike - Spotify playlist with similar songs, according to Last.fm
YOU.DJ - Mix SoundCloud music and Youtube videos live in your browser.
Playlist Machinery - Tools that help you create & organize your Spotify playlists