Back in May 08, I published details of an Arduino powered pendulum. A magnet on the end of the pendulum activates a reed switch when the pendulum is in a particular position. At the right moment, a fixed electromagnet "tugs" on the same magnet, keeping the pendulum swinging. Useful? not. Art! (?)
It generally "just works". For instance, a 12 foot long installation has been "tick-tocking" aesthetically in a stairwell without without any attention for the past month.