Overhead Dumbbell Lunges
No offense to the fellow doing crunches behind me, but if you want to build great abs, rethink the way you train them. Doing endless crunches on a balancing ball is immensely inferior to exercises that force you to stabilize WHILE overloading the muscles of your core/abs.
This is a great variation on a classic and another compound exercise that works numerous muscles. You need to use a weight you can easily clean over your head. Make sure you lock your elbow and keep your shoulder and the weight over the middle of the body. That's why my head is infront of my arm, that's the locked out position, just like in the Olympic snatch.
This builds your glutes, your hams, your quads, and because of the overhead carry your core and your shoulders.