RPN Calculator is a free, open-source, and cross-platform calculator app for almost all popular Linux distributions as well as macOS and Windows 11.
RPN stands for Reverse Polish Notation, commonly referred to as the reverse Łukasiewicz notation (also called postfix notation). It's a lesser known mathematical notation in which operators follow their operands (as opposed to the prefix or Polish notation).

RPN Calculator (formerly known as the x11-calc) is basically a comprehensive simulator app for a series of oldschool HP calculators written in C using X11.

The simulator is capable of bringing to your desktop some various calculators from the HP Classic series (HP 35, HP 80, HP 45, and HP 70), the Woodstock series (HP 21, HP 22, HP 25, HP 27, HP 29c), and the Spice series (HP 31e, HP 32e, HP 33e, HP33c, HP34c, etc.).

Other supported series include the Voyager series (HP 10c, HP 11c, HP 12,c, HP 15c, and HP 16c), as well as the "non-series" HP 67.

Using RPN Calculator (or x11-calc) might be a bit cumbersome at first since all it does is provide you with various old-school GUIs (very accurate to the real things, I might add).

To change between all the supported calculators, simply right-click on the app's icon and select "Set Default Calculator." Please note that this is only possible on some desktop environments. Two good examples include GNOME and KDE. For the rest, you may have to resort to using the command line.

The thing about RPN Calculator is that it does require you to do a bit of reading before you get accustomed to all its functionalities and quirks. For instance, the app heavily relies on keyboard shortcuts for most of its features. CTRL+Z exists the simulator, CTRL+C resets the simulator its initial state, and CTRL+T toggles the tracing of the simulator code execution.

A full list of keyboard shortcuts, as well as a list of all supported desktop environments and other very useful configuration information is neatly organized on the app's official GitHub repository.