Calca!

Mathematica is a computational software program used widely in scientific, engineering, and mathematical fields and other areas of technical computing.

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.

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+).