I have posted on here before about my free jam tracks site (they are literally free, no advertising or anything - just for musos to enjoy).
http://www.freejamtracks.com
I put up some Country, jazz, funk, reggae and blues over the weekend.
And I've dropped the bass out for bass players here:
http://www.freejamtracks.com/tracks4bass.html
Enjoy,
Nick.