Spotalike - Spotify playlist with similar songs, according to Last.fm
Parallel - Listen to music with friends over Spotify at the same time
MusaicFM - An iTunes like screen saver based on Spotify and Last.fm
Slackbox - Spotify playlist collaboration through Slack
Bowtie - Bowtie is a desktop accessory that allows you to see and control your music (iTunes, Spotify...
Join My Playlist - Listen to music live together with your friends in Spotify.