My Breitling watch has a built-in circular slide rule. When in a foreign country, I set it (once) to the local currency exchange rate, and can quickly convert how much anything costs into dollars. I could do the same with the calculator on my phone, but I would have to key in the exchange rate every time - if I can even remember what it is.
I've got a circular slide rule (not really a rule, is it?) with square and cube values, so you can convert surface and volume. Conversion numbers are printed on the back. It seems to have been made as a cheap handout. Not that I ever use it, but it's a cool idea.
