OFFICIAL PAGE: MAZTR
MAZTR offers an online Audio Stem Separator service. Free users can upload up to one 5MB mp3 file per day, separate the audio into 2 stems (vocals and instrumentals) and have the results returned in mp3 form. Pro (Paid) Account users can upload unlimited 100MB files, separate them into 2 stems (vocals and instrumentals), 4 stems (vocals, bass, drums, other), or 5 stems (vocals, bass, drums, piano, other) and can download the output as WAV files or 320kbps MP3 files. Full details at the above link.
Screenshots courtesy of MAZTR