If you happen to have a chance to visit Bletchley Park do not miss the opportunity to visit the National Museum of Computing where you will be able to see a large collection of computers of all sizes and ages. A recent addition is the Harwell Dekatron / WITCH which came back to like on November 20th, 2012.
The Harwell Dekatron or WITCH is the World’s oldest original working digital computer dating from 1951. WITCH is an acronym that stands for Wolverhampton Instrument for Teaching Computing from Harwell. The computer aquired this name when, in 1957, it was offered in a competition to an educational establishment. The competition was won by the Wolverhampton and Staffordshire College of Technology.
The machine uses “dekatrons” for its volatile memory (think of is as RAM) and it works on a decimal system, as opposed to the binary. The dekatrons are visible and thus one can literally see the state of the memory when the machine is operating. This sounds great when trying to explain how a computer works!
More information can be obtained here