Playlist Machinery - Tools that help you create & organize your Spotify playlists
Magic Playlist - Get the playlist of your dreams based on a song
Playlistor - Convert Apple Music playlists to Spotify and vice versa
Spotalike - Spotify playlist with similar songs, according to Last.fm
playAppleMusic.com - An (unofficial) web player for Apple Music
FreeYourMusic - Migrate from and to any music services