Magic Playlist - Get the playlist of your dreams based on a song
Twinjack - Stream Spotify music with anyone with just a link
Spotalike - Spotify playlist with similar songs, according to Last.fm
BeatSense - Discover great music hand-picked by real people, in a single click. FREE and in real-time!
Playlist Machinery - Tools that help you create & organize your Spotify playlists
ReverbNation Discover - Stream hand-picked, ad free music from emerging artists.