Arduino BNC
This is a binary numeric clock I made as a diploma project for my Electronics Technician degree. The case is made using LEGO, and the brain is an Arduino Duamilanove.
The timekeeping is based on the crystal the arduino uses, which has the accuracy of 100ppm. 100ppm accuracy dictates that the error is around 8 second per day. While this can be compensated, as the error doesn't change too much, it does mean that the clock will never be totally accurate.