<< Squats and lunges. Pretty much any non-machine quad/hamstring excercise. >>
Squats are the king of all exercises. Doing them works EVERY muscle below your waist, your butt gets it's fair share, as well as your quads.
Do them properly, and with a spotter(s) if you need it, squats are also a quick way to really screw up your back and knees doing them improperly.