If you like calculators, you’ll like PCalc. It’s everything that Mac OS X’s calculator is not.
A fully featured scientific calculator with support for hexadecimal, octal and binary calculations, as well as an RPN mode, parentheses, programmable functions, and an extensive set of unit conversions.
9WR – PCalc: Scientific calculator; RPN, programmable, hex, octal, binary calculations; iPhone, too.