The Antikythera mechanism is a mechanical computer. The amazing thing about it is that it was probably built about 150–100 BC. and it is utterly unique; there are no known earlier examples of a similar technology, and artifacts of such complexity did not reappear until the 14th century, when mechanical astronomical clocks appeared in Europe.
Dad spotted a documentary on the mechanism on BBC4 the other day. Having been interested in it for years, I assumed the programme wouldn't tell me much I didn't know already. How wrong I was!
It gave an absolute wealth of detail about it, and along with the stunning graphics, I would say it's the best programme on the subject.
Here's what boffins now believe it looked like originally:
See the full programme here: