This is a more capable calculator than the one that comes with Mac OS X.
It has all the standard functions, plus it includes many pre-defined constants, support for using variables, and a command history. You can choose hex, octal or binary input and output. An expandable expression entry field evaluates expressions using the standard order of operations.
This also comes in a command-line version that works on most forms of Unix (regularly compiled for Linux x86 and PPC).