Symmetricom’s SA.3Xm miniature rubidium atomic clock module is about 1/5th the size of the smallest atomic clocks which are currently available. It’s designed for use in applications where accurate timing is critical, like high speed wireless data services Although I suppose you could make a really precise desk clock with it too if you have an OCD about being on time.

For those of us who don’t need such accuracy, there’s always this clock.