The Antikythera Mechanism. Source
This old, decayed machine is perhaps the most amazing relic of history. Discovered over 100 years ago aboard the Antikythera wreck, this device is an extremely accurate astronomical position calculator. It was designed to locate the position of stars and planets in the night sky given a date and time. Of course, we have things like this everywhere now, but what is fascinating about this specific machine is that the technology is thought to be over 1,000 years ahead of its time. The complexity and craftsmanship that went into this book-sized computer is astounding.