OFFICIAL PAGE: Melody ML
Melody ML is a web-based service for Demucs. It allows you to upload a source file in MP3, Wav, Flac, or Ogg/Vorbis form which will be separated into 2 or 4 stems. The service is currently offered free of charge for two songs after which each song costs 50 cents. Full details at the link above.
Screenshot courtesy of Melody ML