I think www.guitarbt.com is the answer to this thread. They have lots of backing tracks of actual songs and also lots of random jam tracks to play around with
if you have sharing software (kazaa, emule, limewire) you can download "guitar pro 4" and the "guitar pro 4 patch update" from it and then go to www.mysongbook.com and download any song you want (from country to metal). you can disable guitars, bass or drums, and play over the original solos or invent new ones for your favorites songs, or create a new drum beat and bass line and follow it to your taste