Spotalike - Spotify playlist with similar songs, according to Last.fm
BandNext - Discover bands that sound similar to artists you already love with BandNext. A single click saves your results to a Youtube playlist.
MusaicFM - An iTunes like screen saver based on Spotify and Last.fm
Spotify - Map shows when two people play same song at same time
Bowtie - Bowtie is a desktop accessory that allows you to see and control your music (iTunes, Spotify...
stats.fm - With the click of a button you'll be logged with your Spotify account and you'll instantly gain access to a valhalla of cool stats and insights.