Enigma/Paper Enigma - Franklin Heath Ltd Wiki

Link: http://wiki.franklinheath.co.uk/index.php/Enigma/Paper_Enigma (via shareaholic.com)

Although the Enigma cipher as a whole is quite complex (it's complexity is comparable to modern cryptographic algorithms) the individual transformations within it are relatively simple. In fact, they are simple enough that you can decipher an Enigma message with just a paper model.
Our model is three-dimensional, to illustrate the wiring of a real Enigma machine. It needs only common household items to construct, and is completely compatible with all settings of a real Enigma machine (models I, M1, M2 and M3).