It always bothered me that the calculator application that comes with Windows did not show the equation that you had typed in and had no user variables. This application is a simple calculator that aims to fix a few of these things.
The calculator shows the equation that you've typed in and automatically evaluates it as you make changes. Variables A through Z (capital letters) are available to define and use in your equations. Simply-double click a variable's label on the left to define it and double click its value to insert it into the current equation.