A simple, modern calculator that uses RPN.
Reverse Polish Notation is a system where the operators are entered after the operands.
For example, rather than 2 + 2, you would enter:
2
2
+
The main benefit is a faster, more accurate interface:
Reverse Polish calculators do not need expressions to be parenthesized, so fewer operations need to be entered to perform typical calculations. Additionally, users of reverse Polish calculators made fewer mistakes than for other types of calculators.
(from the Wikipedia article above).