OFFICIAL PAGE: VOXISO
VOXISO is a web-based service for separating your audio. According to their website, "The basic plan is used for vocal isolation which removes the instrumental and leaves only the acapella. The advanced plan is used for vocal and instrumental isolation which provides the user with both isolated vocals and instrumental tracks." The service accepts MP3, WAV, FLAC, WMA, OGG, AAC, AIF, MPA, MPC, OGA, IFF, M4A, M4B files and many more. The service is offered with several subscription plans and the maximum track length is 10 minutes regardless of subscription plan.
Screenshot courtesy of VOXISO