A really great book [1] which details how Marian Rejewski, Jerzy Rozycki and Henryk Zygalski exploited a lot of these human errors and managed to keep up with the changes of the Enigma machine.
which is outside the NSA headquarters. They have old Enigma machines, which are quite compact, about typewriter size, as well as a decoder, which is more like a bank of refrigerators. And a lot of other artifacts. It seemed to be mainly a labor of love by retirees and crypto geeks, rather than some officially compiled museum. Rather touching, in a surprising way.
[1] If you can find it somewhere for a fair price, http://www.amazon.com/Enigma-German-Machine-Foreign-intellig...