C# Programmable Calculator
Historical article · Originally published November 6, 2003; technical details may be outdated.
How cool is a calculator that you can actually program what each button does using C#? Look for yourself:
“C# Programmable Calculator is a Reverse Polish Notation (RPN) calculator with custom buttons programmable in C#.“
Its shareware, but the unregistered version still gives you 20 programmable buttons. I'll never need more than that :)