The "prone jackknifes" on the balance ball are my favourite ab exercises. Dive onto the ball until your knees are on the ball, your hands flat on the floor, then draw your knees up to your chest.

Also, you can do planks on them and roll around the ball with your elbows while doing it.

Yes, I fell off the ball quite a few times. *G*

It's important that the balance ball is the right size, though.