Frink!

SpeedCrunch is a free, fast, high precision open source algebraic calculator with an intuitive interface and a extensive list of features, including: Unlimited Variables, Syntax Highlighting, Functions, with 'Automagic' Completion, 50 Decimal Precision, Input History and more...

Qalculate! is a multi-purpose desktop calculator for GNU/Linux and Windows. It is small and simple to use but with much power and versatility underneath. Features include customizable functions, units, arbitrary precision, plotting, and a user-friendly interface (KDE or GTK+).

Maths on a Mac as it should be. 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.

OpalCalc is a brand new type of calculator for the PC. As easy to use as Notepad or a word processor, OpalCalc allows natural language in the sum and multi-line support so you can go back and adjust your previous calculations on the fly.