RPN Calculator от Mike Conca

This extension adds a convenient reverse Polish notation (RPN) calculator as a browser action button. Also called postfix, RPN calculators take their operators after the operands.

For example, to get the sum of two and three on a normal calculator, you would hit "2 + 3 =". On an RPN calculator, you would hit "2 Enter 3 +". In general, RPN calculators require fewer keystrokes over a series of extended operations.

It may seem strange at first, but once you get used to it, you'll never go back. HP made a series of extremely popular RPN calculators back in the 1970's and 1980's.