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
Music Suggestions Ninja - Enter an artist's name & find similar, lesser known artists
Bowtie - Bowtie is a desktop accessory that allows you to see and control your music (iTunes, Spotify...
Boil the Frog - Create a seamless playlist between any two artists