Ball Crunches-Abdominal Exercises

Here's a good video demonstration and explanation of crunches on a stability ball.  As explained in the video, the advantage (vs crunches on the floor)  is the ability to get a better range of motion on the abdominal muscles.   I think you may be able to get more reps than they discuss in the video, but I agree with them on emphasizing quality repetitions when you train. 

One final thing:
As I always say, abdominal exercises are important, but they don't have much to do with whether or not you have a lean stomach or the "six pack" look.  In order to get that look you have to get lean.  I'd recommend a diet and exercise program like Truth About Abs (Mike Geary) if you are looking for a flat stomach.