Slackbox - Spotify playlist collaboration through Slack
Spotalike - Spotify playlist with similar songs, according to Last.fm
Playlist Machinery - Tools that help you create & organize your Spotify playlists
MusaicFM - An iTunes like screen saver based on Spotify and Last.fm
PlaylistMap - Connect with Spotify playlist curators from around the world
Bowtie - Bowtie is a desktop accessory that allows you to see and control your music (iTunes, Spotify...