Openwhyd - Collect and Share the tracks you love.
Magic Playlist - Get the playlist of your dreams based on a song
Zoff - Collaborate on a live music and video playlist with your friends.
Spotalike - Spotify playlist with similar songs, according to Last.fm
SoundClick - SoundClick is a music-based social community.
Playlist Machinery - Tools that help you create & organize your Spotify playlists