I'll tell you of a great pushup exercise. You need a bosu ball (one of those 1/2 balls with a flat surface) With ball side down, place your hands at wither end of the flat surface. Do the pushup with one leg in the air x 10, then with the other leg in the air.
Next with hands on the floor, place feet on a pilates ball, then do the inverted push-up.
push-up's and core work. two great exercises, one great result.