Magic Playlist - Get the playlist of your dreams based on a song
Bookmark OS - Bookmark OS is like Mac or Windows optimized for bookmarks.
Spotalike - Spotify playlist with similar songs, according to Last.fm
Siimpl Bookmarking - Start collecting and organizing what interests you
Playlist Machinery - Tools that help you create & organize your Spotify playlists
Annotate.tv - Optimize your learning on YouTube