rpCalc is a feature-rich Open Source desktop calculator.
rpCalc started out as a little program aimed at trying out various Python GUI toolkits. But the author ended up using it all the time noting that "it's much quicker to pull it up than to pull an actual HP calculator out of the desk." rpCalc now includes several improvements over that initial little program, so at that point, the author decided to make it available to others who also like RPN calculators.
Uses reverse polish notation, similar to most Hewlett-Packard calculators
The number, operator or command text on any key can be typed, or the mouse can be used to hit the key
If desired, the four RPN registers can be shown on the main display
A separate window can display the four RPN registers, a history of recent calculations, or the contents of the ten memory registers
A separate window converts to and from other number bases (hexadecimal, octal, and binary)
Any values from the extra windows can be copied to the calculator display or to the clipboard
Options can be set to control the display of numbers and the initial window configuration