The Antikythera Mechanism
an Astronomical clock from ~100 – 200 BC found in a shipwreck near the island of Antikythera.
Optical Lattice Atomic Clocks
Proposed in 2001 by University of Tokyo Professor Hidetoshi Katori, these are very much more accurate than standard atomic clocks. They are accurate enough to measure relativity and geologic density. Experimentally demonstrated in 2005.