OFFICIAL PAGE: Demixer
Demixer is a High Quality Online Vocal Remover Service powered by the advanced audio separation software at AudioSourceRE. You can upload files in WAV, MP3, or MP4 formats with support for up to 16bit 44.1 kHz audio. You have the option of two stem (vocals and backing) or four stem (vocals, drum, bass & instrumentals) separations. The online mixer allows you to hear the separated results in the context of a stereo mix with level and panning controls for each stem. Several pricing options are available for the service. Full details at the link above.
Screenshots courtesy of Demixer.com