Magic Playlist - Get the playlist of your dreams based on a song
Plug - Lightweight OSX player for Hype Machine
Spotalike - Spotify playlist with similar songs, according to Last.fm
Playlist Machinery - Tools that help you create & organize your Spotify playlists
Playlist Hunt - Playlist Hunt is one of the best applications for Apple users to get the best music through it.
Spotify - Map shows when two people play same song at same time