This should prove to be one of the more esoteric add-ons you'll find for World of Warcraft, and most people won't care for it. Luckily, you're not most people.
This is my implementation of an RPN (aka Hewlett-Packard) style calculator. It's not often one needs a calculator, but since I usually hate to alt-tab away from what I'm doing I went looking for a calculator addon. I found a very fine one, called myCalc by Koraxen, but being an electrical engineer I really wanted to use an RPN calculator. This was eccentric enough that I knew I had to make my own.
For those who are curious, RPN stands for Reverse Polish Notation and is the most intuitive form of calculator entry. Its four register stack also allows for some very complex formulas to be keyed in. You can usually recognize an RPN calculator by its lack of an 'equals' key.
To install: Place the unzipped RPNCalc folder in the <your path>\World of Warcraft\Interface\Addons\ folder.
To show: After logging on, type /rpn in the chat area to show the calculator.
To hide: Click the X button or type /rpn again.
1.1.0 – 10/12/2010 - Update, changed TOC for version 4.0 of the World of Warcraft client.
1.0.1 – 06/15/2010 - Bug fix, button "Cl x" was allowing the stack to be pushed a second time after clearing the x register.
1.0.0 - 12/08/2009 - Initial release for version 3.3 of the World of Warcraft client.