For those of us who don't have the knees for running, here is a list of good cardio workouts said to give the most bang for the buck:

1. Indoor cycling
2. Circuit training or HIT training.
3. Versa Climber (or other form of climber)
4. Plyometrics
5. And lets not forget a good old fashion aerobic routine set to your favorite music!

Cardio is key to loosing weight, but you must get your blood rate up and keep it there for 20 minutes or more to have optimal effect. So after your warm up try to get your heart rate up and then maintain it without letting your heart rate go too high. Learn the two finger method and you can count your beats in between moves.