centre for digital music
Music Information Retrieval
ccMixter Creative Commons multitrack resource.
My Life in the Bush of Ghosts A couple of Brian Eno and David Byrne multitracks.
Real World Remixed This site allows you to download 'sample packs' - multitrack recordings from Real World Records and Peter Gabriel. I don't think the site has full, time-aligned stems unfortunately. Instead, you can create new mixes from the sample packs.
Karaoke Version This site sells multitrack versions of a lot popular western rock & pop chart music.
remix.nin.com Multitrack stems from Nine Inch Nails.
ninremixes.com More multitrack stems from Nine Inch Nails.