I recently had the opportunity to visit the historic Bletchley Park, home of British codebreaking efforts during the Second World War. Aside from the military historical significance, this mansion surrounded by temporary sheds is also important to the history of computing. It was there that several of the earliest computers were built, including the famous Colossus codebreaker. It was also where Alan Turing was stationed and worked on the new practical applications of his hitherto theoretical discipline of Computer Science (not that they would have called it that at the time).
Bletchley Park is also the site of the National Museum of Computing, and I was able to take some video footage of the fascinating machines they have there.
Colossus was the first (partially) programmable digital electronic computer, and was used to break the German High Command's encrypted messages during the Second World War. The rebuilt version is magnificently mechanical, with constant whirring of spinning wheels of tape, clacking of relays, analogue dials and oscilloscopes hooked to it, and a powerful wave of heat emanating from the wall of valves.
This next video is of the Harwell Dekatron (a.k.a. the WITCH — Wolverhampton Instrument for Teaching Computing from Harwell). This carefully rebuilt machine is now the oldest still-working original digital computer out there.
Dekatrons are awesome counting devices whose operation is visible in the mesmerizing spinning light pattern they emit. The WITCH uses them for intermediary memory storage, and you can even see a few spinning around in the video if you look carefully.