A free, full-featured, graphically laid out, high-precision, scientific calculator for Mac OS X 10.4 and greater. Full source-code is included with the distribution.

Ideal if you need to enter large expressions or have accurate precision. "Data" drawers allow an easy way to generate statistical data, linear regression and gaussian elimination. The extensive support of complex numbers and hexadecimal numbers is also a significant benefit for anyone who has to work with this type of data.
Further features include:

25 accurate digits of precision

Complex numbers

Hexdecimal, octal, binary, decimal and 2's complement display.

Floating point numbers, even in non-decimal radices.

A full expression history (go back to anything)

A graphical display that you can click on to change the entry point

Value memory limited only by computer memory.

Statistics functions

Linear regression

Matrix functions including gaussian elimination, inversion and determinants.

Large number of scientific constants built-in.

Source code includes:

100% Cocoa goodness.

Some documentation in the help (so you have somewhere to start)

Quick guide to adding/changing constants and functions

A full "BigFloat" implementation for complex numbers