Calculator.NET is an enhanced version of the standard calculator in Microsoft Windows. It uses RPN (Reverse Polish Notation), which is used by many pocket calculators, e.g. from Hewlett-Packard. If you have used a HP 11/28/32/41/42/48, or any other RPN calculator, you will feel right at home.
Some of the enhancements included: RPN, extended function set, and customizable user interface.