OFFICIAL PAGE: Conversion-Tool
Conversion-Tool provides another free web-based service for separating your audio into 2 stems (Voice / instruments), 4 stems (Vocals, drums, bass, other instruments), or 5 stems (Vocals, drums, bass, piano, other instruments). There is no description of the processing utilized. MP3, FLAC, and WAV formats are supported as well as the ability to import audio from many video portals for processing.
Screenshot courtesy of Conversion-Tool.com