Of course, I am referring to work that is shared with the permission of the artist/publisher etc. Like for instance Brian Eno had the stems to some of the Bush of the Ghosts album (to remix) up a while ago, though that doesn't seem to be available any more. I guess what I'm after is similar situations where artists have made available their recordings in perhaps a remix competition or something like that. I'm definitely not after anything illegal.

I've seen these from time to time, but I'm not aware of any specific site that provides them.

Perhaps if you did a google search using terms like "audio stems", "remix", etc., you might find a source.

http://minusdrums.com/ has some drumless tracks, but not free.
http://drumlesssplitsongs.com/
http://www.groovedrumming.com/

Thanks everyone!

Trent Reznor has published a lot of Nine Inch Nails' catalogue as free stems. I've played with a few, and think mixing pro recordings is a great way to hone chops.

http://www.daveweckl.com/grooveme.htm

Peter Gabriel had some of his a year or two ago