Algodoo

That's awesome. Could you explain the math/code background of it?

See Two Synthesis Methods for Non-Circular Cylindrical Gears for the math. Save the scene with a .phn extension and then open it with a text editor to see the code. The simple explanation is that you create a shape and a hinge and then the second shape must have a surface that rolls without slipping on the first shape and must have a perimeter that is n2/n1 times the first shape's perimeter.
The circle teeth spacing should be the length of one side divided by an odd number. The phasing of the gear teeth of the first gear with respect to the second needs to be correct for smooth operation.