Orpie is a fullscreen RPN calculator for the console. Its operation is similar to that of modern HP calculators, but data entry has been optimized for efficiency on a PC keyboard. Features include extensive scientific calculator functionality, command completion, and a visible interactive stack.

Calca is a powerful symbolic calculator that gives you instant answers as you type. It solves equations and simplifies complex expressions… It’s also a rich Markdown text editor so that you can explain your ideas alongside your calculations.

qc is a command line tool that calculates in-line math expressions with no pipes or command line tricks. The result of the calculation is displayed in the the command line and automatically copied to the clipboard for easy of use.

A handy way to use Python’s maths library at the command line.

Soulver is a new kind of calculator application which uses a simple yet powerful word-processor style interface instead of the traditional “button” approach to doing math.

Update 21/04/10: As itohsnap pointed out on Twitter, there’s one thing wrong in that screenshot. In Soulver’s defence, it gets the right answer if you tell it to do k / q. The fact I didn’t notice is why I need a calculator in the first place.

"bc is an arbitrary precision numeric processing language." I’m not even sure what that means, but can report that bc also works well as a command line calculator for basic arithmetic.