LC-2 is a long running Lego clock.  It'll run for around 27 hours off a 6 foot drop. 
It requires 5lbs of weight to power it.  This clock is COMPLETELY mechanical and 
requires no electricity.  The RCX is used to drive a "rewind" motor.  
I got tired of rewinding it by hand.

My other clock and further details can be seen on:

www.trofemuk.org

I'm attempting to build a striking mechanism now and I've started working
with the large Hailfire Droid wheels in hopes of getting more runtime.