Hamstring curls with a stability ball are one of my favorite hamstring exercises.
Lye flat on the ground with the stability ball beneath your heels. Lift your glutes and back up off the ground to make a streight line with your legs and back while keeping your neck relaxed on the ground and hands flat on the floor. Now bend your knees and curl the ball towards you while keeping the back and glutes tight, alligned and lifted the entire time then return to the starting position. I like to do these at the end of my leg days. If your doing them right you will definitly feel the bur.