OFFICIAL PAGE: Song Peel
Song Peel is a online vocal remover service that uses proprietary methods to separate the vocal and instrumental elements from a mono or stereo source file for a nominal fee. Various pricing options are offered and a 30 day money-back guarantee is provided. Once the process is complete you are provided with two uncompressed wav files, the instrumental portion of your supplied file and a vocals only version. I was quite impressed with the results from the two fairly difficult mono tracks that I provided to Song Peel!
Screenshot courtesy of songpeel.com