I've been borrowing a DVD from a co worker, and I really like it. Its called Crunch - Fat Burning Dance Party. Its around 40 to 45 mins, and I think its fun. It was hard the first couple of times, but its easier to follow now. For me, its a challenging DVD, but I'm not in the greatest of shape, so it may be easier to others.
I also have Denise Austin - Shrink Your Female Fat Zones. Its a 40 min workout with weights and the stability ball, but its split into 2 20 min segments. Since I like to sleep in a bit, I tend to do the first 20 mins one morning, and the second half the next morning. I like this one too, its fun, easy to follow, and I think its a decent work out. I know others have commented on it, saying its pretty basic and easy work out, but it just depends on your fitness level I guess
Hope that gives you some ideas anyway
~Liz