I purchased three before I got pregnant. They all seemed ridiculously easy when I tried them out pre-pregnancy, but now that I am pregnant, they seem to be at an appropriately challenging level (I was very active beforehand).
1. Prenatal Fitness Fix by Erin O'Brien - this is a 2 DVD set (prenatal and postnatal). The exercised don't require any "equipment" other than your living room/dining room furniture (couch, coffee table, firm chair) and a towel or mat for the floor. Erin is very down to earth and encouraging. The workout is 45 min and I like it a lot. I would say this workout is pretty low-impact while still being up-tempo.
2. Lindsay Brin's Complete Pregnancy 4-DVD Workout Set: Cardio, Toning PLUS Yoga - This set has a separate DVD for each trimester plus a postnatal bootcamp DVD. While I like the exercises in this program, and the fact that the workout changes for each trimester, I find Lindsay a little fake/annoying, but it's not so bad that it keeping me from doing the workouts. Maybe she's just too cheery/peppy for me. There are toninig, cardio, and yoga sections on each disk so you can choose to do some/all of them as you want. Doing everything at one time take just under an hour. You will need hand-held weights and a mat for the floor.
3. Suzanne Bowen's Long and Lean Prenatal Workout - This DVD is about 50 minutes long and is very fluid/low-impact. You just need a mat for the floor. This workout definitely tones the muscles and helps you get a good stretch, but doesn't have the cardio aspect that the other two have. Still a really good workout, and I enjoy it. The music is very soothing and Suzanne's voice is very calm and relaxing.