http://www.myguitarjournal.com/2010/01/28/boss-dr-880-dr-rhythm/
Its not cheap, somewhere just under $500, but I feel its a great value for practice & improvement. I've also tried GuitarPort, which is only $99, and works off my Mac, but its not as instantaneous as the DR-880. However, there are free jam tracks out there that you can use with GuitarPort, if you search around enough on-line, and some are very good. Just a few options.