DickB over at the 430h forum built this cool MSP430 propelled pendulum clock:
This pendulum clock is not only regulated by the pendulum, but it is driven by it. A hidden magnet in the pendulum swings past a hidden coil and induces a current. This is detected and then a current pulse is fed into the coil, creating an electromagnet that repels the pendulum to keep it moving.
Via 43oh.