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
Magic Playlist - Get the playlist of your dreams based on a song
Bowtie - Bowtie is a desktop accessory that allows you to see and control your music (iTunes, Spotify...
Listen for Mac - Add songs to Spotify playlists using keyboard shortcuts