The Programmer's Calculator.
Supports a rich expression format, which is in most cases identical to what you would type in your programming language.
Support for adding unlimited numbers of variables.
Built-in debugger that lets you see the expression simplified step by step.
Debugger Back-step lets you backup as many steps as you like so that you can replay critical operations.
C/C operator precedence
Mouse optional, all calculator commands and functions can be done using the keyboard alone.
Mini-Calc Mode lets your calculator take up very little screen real estate, and yet remain entirely usable.
Numerous built-in unit conversions (memory sizes, lengths, areas, volumes, temperatures etc.)
Built-in great circle route and straight line distance calculators.