One giant 'water wheel', but instead of water, use the zombies. Herd them into a chute at the top, and they step onto the paddle. They ride the paddle to the ground, just like water would, turning the wheel. When they reach the ground, they get dumped out. Jut herd them back up to the top, and keep the cycle going. The wheel drives whatever device you wish - generator, grinding mill, saw mill, whatever.
No real intelligence necessary, just an unlimited supply of zombies who want to do nothing but walk aimlessly in a herd.
EDIT
Just as a reminder, the energy that you obtain out of the zombies is the energy they expend in climbing back to the top of the cliff or structure, and then lose again in their descent down the wheel. You would get even more energy if you had them carry rocks or something up to the top, and take them down the wheel.