Mechanical Calculators
I love mechanical calculators. You can take them apart and see how they work. Levers press, gears advance, carry mechanisms engage... there are no black box chips obstructing one from appreciating the designer's inventiveness. I enjoy finding ones that don't quite work, figuring out why, and fixing them.
Great Sites for Mechanical Calculators
John only covers mechanical calculators that made their way to Australia, but it really isn't all that limiting a criterion.