Doing your crunches or sit-ups on a stability ball works more muscle fibers than doing those same reps on the floor because your ab muscles are constantly contracting as you curl forward. On an unsteady surface you use more muscles to help stabilize the body.
How to use it: Sit on the center of the ball and lie back so that only your lower is touching it. Raise your torso no more than 30 degrees. To make it harder, hold a five-pound weight plate.