TubeLister - Generate a YouTube playlist from your opened tabs.
Magic Playlist - Get the playlist of your dreams based on a song
DiskyT - Youtube playlists done right
Spotalike - Spotify playlist with similar songs, according to Last.fm
OrganizeTube - Create video playlists from Youtube, Vimeo and other sources
Playlist Machinery - Tools that help you create & organize your Spotify playlists