I do the crunches with the large ball. In addition to different angle mentioned above, it works your proprioceptors (sp?), basically the small muscles that keep you oriented in space - i.e. you have to work to keep your balance as well as doing crunches.

I also do crunches on a bench that you use in a reverse incline with your feet hooked under rolls at the top (have no idea what this is called!). My DH does something called a bridge, an exercise his PT gave him (he needs strong abs as he had multiple compression fractures in his back and needs good muscular support). Anyway, you lay on your back on the floor or a mat, arms by your side. Bend your knees with your heels about 1/2 way to your butt, an angle at which you can lift your hips and be stable. Then "set" your abs, lift your hips and then lift one foot 2-3" off the ground, hold for several seconds and return to the floor, repeat with the other foot. This seems easy for the first couple, then you start to feel it in your abs. It hits some different muscles than crunches.