Excalibur for Windows 32-bit version is a freeware full featured RPN-only calculator. This is a serious calculator for serious users. Excalibur takes up a small amount of resources for all it does so well.
Excalibur does not use more conventional "algebraic" notation like many low-end calculators on the market. Instead it uses the high-powered parenthesis-free RPN notation (Reverse Polish Notation). This notation was named in honor of a Polish mathematician and logician named Jan Lukasiewicz. Polish notation places the operators in an expression before the numbers. RPN places them after the numbers (which is why they call it Reverse Polish Notation)
RPN is really nothing more than the method of entry of numbers and operators into the calculator.