Stability balls are a great tool to increase and maintain core stability and strength. With all my clients that have the support base to move to unstable surfaces I use the ball with any exercise that you would use a bench, in addition to including core specific exercises as well.
My top 5 (favorite) ball exercises:
1. DB Pullover on the Stability Ball
3. SB Hamstring Curls
4. SB Push up to Prone knee tuck
5. SB Abdominal Crunch