Squats are the one of the best exercises for leg development, power, definition and strength, as well as cardiovascular health. Any exercise that forces you to use lots of big muscle groups is going to get your blood pumping and lungs working!
What I will often add to my training routine will be 20 rep squats. I was first introduced to 20 rep squats in John McCallum's book Keys to progress. This book was put together from articles written by McCallum way back in the 60's. Even with all the advancements in training, it is one of the best books i've ever read on gaining strength.
One of his first articles in the book is based on 20 rep squats. These will stimulate your strength gains and growth massively if done with the right intensity. McCallum, states that you must pick a weight you can do only 10 reps with and then do 20 with it. That's how hard you have to work. Every rep past 10 needs to be do or die.
In between reps you can take 3 deep breaths, on the third breath, you hold it and go down for the next squat, breathing out as you go up to the top.
They aren't for the faint hearted but will give you gains throughout your whole body if done with the right intesity.