Don't know if this is exactly the kinda thing that you are looking
for but might be of use to you (and anyone else) all the same.
Main list of artists:
Guitarbt.com - Backing Tracks 1
Styles and non-specific jamtracks:
Guitarbt.com - Backing Tracks 2
There's a lot of good stuff in there, although most of the jamtracks
on the styles page come from those CD's you get with tuition books.