Playlist Machinery - Tools that help you create & organize your Spotify playlists
SicMu Player - SicMu Player is an application that enables users to play audio files on their devices easily.
Playlistor - Convert Apple Music playlists to Spotify and vice versa
Poolsuite™ - The super-summer music player
FreeYourMusic - Migrate from and to any music services
playAppleMusic.com - An (unofficial) web player for Apple Music