The Colossus

The first computer, built by Tommy Flowers for Bletchley Park to crack the high security WWII code. Thousands of valves, paper tape moving at 30 mph, and its existence only revealed publicly in the 90's. Lots of people got together to recall the details and rebuild it.

They also have replica working Bombe machines and lots of Enigma machines at Bletchley Park - well worth a visit.