Genaille-Lucas Rulers are pretty cool.  They are a clever calculator that you can use to multiply some pretty big numbers.

MathHappens summer staff member Anh Nguyen made a colorful set from scratch in Corel Draw that you can download and print on a laser or inkjet printer.  Feel free to copy, explore, and use.  It’s a great project to try to reverse engineer and figure out how they work.  (You can also check the wikipedia for some hints)