What *can't* you do with a balance ball? :)

Hold the ball and do jackknife situps, where you lie flat, ball in hands, and raise the ball and your legs toward each other.

Hamstring curls - lie on your back with your legs on the ball, lift your butt off the ground, use your legs to pull the ball toward you. (I hate these with a fiery passion, but they are better than doing the same exercise on the TRX.)

The name of this one escapes me - same position as above, but you are just raising your hips off the ground and back down again. Hip presses?

Use it as a bench while you do flies or chest press with dumb bells. (Lie with your shoulders/back on the ball and your body in a bridge position, and do the exercises as if the ball were a bench.)

Kneel on it while trying to keep your balance. (I once had a trainer who would *stand* on the ball, but that seems like a quick way to a broken neck to me.)

Brace it between your back and the wall and then do split squats. (Or regular squats, like cetera said.) Alternatively, rest your back foot on the ball and do split squats.

Use it as a chair to do sit-down squats, or one-legged squats.