I recently purchased Chordpulse, and I thought I'd come here and recommend it.
It's an easy way to type in a few chords, choose a style, and have a backing track to rehearse to. Then when you are done with that, you can have one click changes of key and tempo.
I know there are other alternatives out there, but some of them are needlessly cluttered for a practice tool. And some of them are much more expensive.
Chordpulse is $24.95.
http://www.chordpulse.com
[I have no affiliation. I just think it is a very useful, simple practice tool at a very fair price.]
