PCalc is a high-powered scientific calculator that goes beyond the features found in the built-in Mac OS scientific calculator. Scientists, mathematicians, or just serious students will find quite a few bonus features comparable to those found in standalone programmable scientific calculators which PCalc's interface mimics--including quick one-key conversions, constants organized by field (from astronomical to physicochemical), an optional RPN mode that matches modern calculators, and decimal, hexadecimal, octal, and binary modes. You can also write your own functions and conversions, maintain multiple calculators (all with different states), and use PCalc's slightly lower-powered Dashboard widget for quick calculations. PCalc even comes with a virtual tape, so you can look through all your past calculations, and you can resize and skin your calculator with a variety of themes.
This shareware app is also available as a cheaper (and very similar) iPhone and iPad app, and the Mac version has been updated to add more of its features. If you regularly use a scientific calculator, you should definitely check out PCalc, on any platform.