Sliderule

If you know what this is, then you are old.
If you have ever used one of these, then you are really old.
But after China detonates an Electromagnetic Pulse bomb (EMP) over the world and knocks out all electronic devices, i'll be all set and ready to multiply and divide big numbers.
Actually i was really amazed at how cool this was, once i figured out how to use it. For example in this picture i set the slider to line up with the 6.

(click to enlarge)

The back of the sliderule is used for sin, cos and tangents. Again i was amazed at how accurate it was and how easy it was to use.