Spotalike - Spotify playlist with similar songs, according to Last.fm
Momixa - Pick two songs and get the perfect mix
MusaicFM - An iTunes like screen saver based on Spotify and Last.fm
FIEND.fm - "Hot or Not" for music discovery: 2 hrs of tracks in 2 mins
Bowtie - Bowtie is a desktop accessory that allows you to see and control your music (iTunes, Spotify...
Pacemaker - Create and share mixtapes with friends