rpCalc started out as a little program written to try out various Python GUI toolkits. But I ended up using it all the time (it's much quicker to pull it up than to pull an actual HP calculator out of the desk), and I made several improvements. So I decided to make it available to others who also like RPN calculators.
#More Info
See the rpCalc homepage for more info.
There is also an online version, written in JavaScript (see the doug-101/rpCalc-js repository for the source code).