Reverse Polish Notation (RPN) Calculators
When I was in school many many years ago, just about everybody in my field of study used HP calculators b/c of the RPN.
I see some old HP models online but they are pretty pricey (about $60 plus shipping), but they were solidly built. My old one is dying.
I realize that everybody has a calculator on their phone nowadays but once you've used RPN it's hard to go back!