Chordpulse

For a year or so I've found this free computer program fun and useful in creating backing tracks to jam with - for example, a cycle of 5ths to play scales against, or a 1-6-2-5 jazz progression to improvise on.  You can enter any chord progression, tempo and key, and it will play back a nice-sounding backup in a variety of styles, through your MIDI output.  You can save your work in its propietary format.  Here is the link: Chordpulse Lite

(Only for Windows users.  The paid version has more chords and styles, and enables you to save in MIDI format.)