I like the Denise Austin tape. It's just exercises, so if you're looking for something that involves spiritual, mind-cleansing aspects of yoga, it's not a good one. But if you're used to aerobics and want something that's just one step away, it's a good choice. I particularly like the salutation to the sun segment at the beginning. It's an energizing series of postures you can do any time.
i have been using ali magraws "body and mind yoga" for a very long time! i love it, it's not really ali macgraw it's erich shifmann and he's very good. also, for a shorter series i actually like "the joy of yoga" by crunch!