I am a muso from Melbourne Australia and have created some jam tracks for other muso's to use.
They are all free - literally - no advertising on the site etc - it's just for the common good of people who like jam tracks.
They are currently set up for guitarists/lead instruments here:
http://www.freejamtracks.com
and a few for bass players are on here:
http://www.freejamtracks.com/tracks4bass.html
They are all basic blues progressions at the moment.
I will update the site with some new progressions soon.
Have fun.
Cheers,
Nick.