Stability balls are useful in many different ways. Using it as a mechanism for balance makes muscles work harder to do any given movement, therefore increasing the benefit of the movement.
Something I mentioned awhile back in a post titled, "Gettin' Tricky", is using my stability ball for sit-ups with deeper extension.
For the length of one song, sit up tall on your ball with your feet flat on the ground and pressed down firmly. Bend your knees at a 90 degree angle. Place your hands behind your head lacing your fingers together, and keeping your your elbows as far away from your ears as possible.
With control, lean backward as far as you can without altering your body position, and then come forward beyond 90 degrees to your lap.
Keep this backward-forward motion going in rhythm to the song, for its entirety.
Back extension (stomach on ball)- 15 reps
Ball wall squats (ball between back and wall)- 15 reps
Leg raises with ball pass (lie on back, pass between hands and feet)- 15 reps
Ball plank with leg raise (hands on ground, raise one leg at at time)- 30 reps (15 reps per leg)
Ball jack-knifes (use ball as foot support, hands on ground, bring knees to chest)- 15 reps
Russian squats (use ball as foot support)- 30 reps (15 reps per leg)