I was looking for more 20 minute workouts today and came across:
Kickbutt/Fitprime 6 Day Trainer has tons of 20 minute workouts
my favorite is Kickboxing which I did today
Moxie 1 / Moxie 2 both great DVDs and are 20 minutes each. Very energetic mostly cardio workouts.
For more relaxed workouts,
Classical Stretch episodes are about 23 minutes in length. I have season 10 and really like it.