PEMDAS is a scientific calculator with an easy-to-use interface for doing calculations with formulas and variables. Next to the info button on the widget is a '+' button that expands the widget to show the formula and variable caches, which give one click access to everything you've done. You can clear the caches by pressing clear on your keyboard twice real fast (shift+delete for notebook users), or double clicking the clear button on the calculator.
In addition, PEMDAS lets you switch between degrees and radians, has support for percentages and most trig, log, and hyperbolic functions, lets you format the answer in scientific and engineering notation, and has a preference for thousands separators. You can also specify the precision of the calculation, and convert between hexadecimal, octal, binary, and decimal.