How Stuff Works: Atomic Clock  
   Concise history, background, definitions and operational information regarding atomic clocks.
   http://www.howstuffworks.com/atomic-clock.htm

How Stuff Works: Pendulum Clock  
   Concise description using drawings and animation to show the operation of a pendulum clock.
   http://www.howstuffworks.com/clock.htm

Doug's Atomic Clocks  
   Details on Doug Hogarth's experiments with his atomic clock.
   http://www.niceties.com/atomic.html