I am a fan of Reverse Polish Notatio calculators. My father, an engineer, introduced me to them at a young age, and I've been hooked ever since. I own an HP48-GX graphic calculator (and have an emulator for the same on my phone) that I've used since high school. (Some people swear by the TI-83 calculators, but honestly the HP48 series is way better as far as I'm concerned.)
Partly so I could have an RPN calculator on my computer (not realizing that Galculator had RPN mode built-in), and partly to teach myself the basics of using Tk in Python, I created a simple RPN calculator program.
The program consists of two parts: a stand-alone command-line calculator, and a GUI component written using Tk. The calculator has most standard arithmetic options (including trig, exponents, logs, probability, and modulus operations), and also has the usual RPN stack operations (roll, swap, dup and drop).
The source code for the calculator can be downloaded here.
To run the calculator as a command-line tool, execute the rpn.py script. To use the full GUI run the rpngui.py script.