Most folks will be unfamiliar with this, but back in the way back times Bell Labs sold or gave away a small cardboard kit to educators to help them teach computation to students. They called it "A CARDboard Illustration Aid to Computation" or CARDIAC. Described on the Wikipedia page here:
http://en.wikipedia.org/wiki/CARDboard_Illustrative_Aid_to_C... it had a wonderful manual that came with it. This is one of the nicer simulators for it.