The pistol, or One-Legged Squat is an excellent test of balance, coordination, strength and power for your entire lower body. If you can't perform them free-standing, you can assist with a chair or broom-stick for balance
The movement is pretty basic:
1.) Stand on one foot, with your other leg straight out in front of your body, hands to your side or slightly in front of you.
2.) Squat down slowly, attemtping to keep your weight balanced between your toes and heel. You may need to move your hands in front of you for balance.
3.) Hold for a moment, then stand back up. This will take a pretty strong contraction with not only your quads, but also your glutes and hamstrings.
For a good article about the pistol that breaks down the major components and suggests training methods to improve each, check out Power-Athletes Magazine.