Yes, an HP-32SII RPN calculator. I teach math and sometimes have to do some calculations while lecturing. My fingers know how to do RPN by themselves, so I can use the calculator blind while continuing to speak. This can be a good trick to impress students. (They would be much less impressed if they ever saw me try to use a non-RPN calculator and fail at simple things.)

The calculator also comes in handy for tallying marks on exams, and it's much more pleasant to use than a calculator app on the computer. I do have Free42 installed everywhere but I don't use it that much.