The Integer Multiplication Machine
This interactive application presents a model of integer multiplication. As in 'The Integer Addition Machine' the model is based on pulleys that drive a robot along a number line. The model gives an intuitive explanation for the rules governing signed number multiplication, e.g. why a negative times a negative should be a positive.
Note that this pure HTML application is completely self-contained. You do not need to be connected to the Internet in order to run it. All you need is a reasonably up to date browser like Chrome or Firefox.