I just received a copy of Band in a Box MegaPak I ordered. It's a program that will automatically play background in any of a number of styles. You just enter the chord progression and pick a style. I figure it be good for practicing. I sprung for the MegaPak 2005 with a huge amount of styles as an achedemic web outlet had it for a discount with a student id.
Took about five minutes to install. It then took a minute or two to start up and put in the chords for one 16 progression. I like programs where you can get the basic to work without reading anything.
I put in a bluegrass waltz progression I will call "White Peace Bird". I tried a number of different waltz styles a few different BG and country styles. All sounded more than good enough to use for practice.
I think this program will be way fun and educational. You go to a jam and you maybe get one 16 bar break on some song. You can loop this thing and solo to your heart's content.
One thing I plan to do is save all the songs I'm working on as MP3s in various speeds so I can put them on my portable player for practice.
Follow the link for sample output in .mid and .mp3 for "White Peace Bird" in E Major 150bpm about three minutes, ten times thru the loop.
E E A A
E E B B
E E A A
E B E E
The mids are really small. I don't know if anyone makes an mp3 player that playes mids. Still with mp3s, I could load all my practice songs on my little 64meg player at a few different speeds.