A CPU you can walk inside :) I was at Bletchley Park this weekend fixing old BBC Micros for the museum. It's a great place for geeks, but also important because it educates people (especially children) about what really goes on in their magic box/tablet of tricks.
i was there last month. the EDSAC is pretty much in the same condition the video. ie not operational yet.
There's a cool tic-tac-toe program for ESDAC called "OXO". you can run it on the emulator.