This one comes from Eric through the mailbag:

http://www.personalmicrocosms.com/html/cspcalc.html

"If you like Reverse Polish Notation (RPN) calculators and you like to program in C#, use C# Programmable Calculator to create custom functions using your favorite programming language. C# Programmable Calculator includes pre-programmed functions in the following categories: math, trigonometry, computer math, finance, and date and time manipulation.

Additionally you can write your own functions in C# using the integrated source code editor. C# Programmable Calculator is distributed as open source."