(Click for a larger image)
The Faber-Castell 67/87 is a plastic 6-inch simplex pocket rule with the Reitz scale arrangement and extended, self-documenting scales. It's a nice little rule, and that's before you discover its secret: flip it over, and there's a 6-digit addiator on the back!
The addiator is a mechanical adding machine, operated by a pen-like stylus, and it allows you to perform addition and subtraction, something slide rules just don't do very well.
The back also has a few conversion factors, the quadratic formula and some trigonometric identities printed on it.