Here are some of my favorites (along with some misc. thoughts):
For stretching without working too hard:
Rodney Yee's Yoga Conditioning for Athletes (Great hip openers and spinal stretches done sitting on a chair, then a short yoga workout w/out the chair, followed by some great seated stretches)
Rodney Yee's Yoga for Energy (Great stretching, no chair, 5 different short workout sections, some move faster than others).
Rodney Yee's Power Yoga for Beginners - Flexibility and Stamina (20 minute intermediate power yoga workouts that move quickly, stretch you out. You'll probably work up a sweat, but they definitely won't wear you out.)
Power Yoga that may wear you out:
Baron Baptiste's Power Yoga Level 2 (Long, 70 minute workout with a lot of lower body work. A lot of balancing and lunge type poses).
Eoinn Finn's Power Yoga (Haven't done this all the way through yet, but it looks great. It keeps moving at a good pace & has beautiful scenery)
David Swenson's Short Forms (Great DVD with a 15, 30, or 45 minute power yoga workout. They are all pretty much the same except that the longer workouts tack on some extra poses. Good pace, beautiful park scenery, and Swenson's voice is relaxing. This follows the usual set of poses, not exactly sure what its called, but its the same series as a lot of other yoga videos.)
Great all around yoga video:
Ali MacGraw's Yoga Mind & Body (Starts with cat back arches and arranges things in a different way. Great desert scenery and great music - if you like Dead Can Dance. This one is different, and it moves at a good pace. You'll probably work up a sweat, but won't kill yourself with this one.)
Laura