Seriously, the Curta is a handheld fully mechanical calculator invented during the 1940s by Curt Herzstark, a prisoner at the Buchenwald concentration camp. It can perform the four basic mathematical operations along with square roots with a little more difficulty.

These things were the best calculators around until electronic ones came out, and they now sell on ebay for over $1000. They were the first things Liechtenstein, the world's richest country (per capita), ever manufactured. Nothing is cooler than the Curta.