Frink - Frink is a programming language and calculating tool designed to help you make real-world calculations.
Frink is different than other calculating tools in that it tracks units of measure through all calculations. Thus, you can add feet and meters, multiply by grams, divide by pounds, and Frink does the right thing through all calculations. It also does currency conversions, date/time math (so you can find how many days to Christmas, which is admittedly easier to do in December...), find historical buying power of the U.S. dollar and British Pound, translate between several languages, and more.
Frink also handles arbitrary-length integers, rational numbers, arbitrary-precision floating point numbers, and complex numbers.
Frink isn't just a calculating tool; it's also a full-featured programming language that lets you write, edit, load, and save programs on your handheld device. Just choose "Mode | Programming" from the menu bar.