YorickMicrosoft Windows

Apple OS/X

iPhone/Touch
Mathomatic ($1.99 at iTunes stores) is a port of the desktop-based Mathomatic system adapted to the iPhone touch interface.

Linux and Unix
Yacas Computer Algebra System
YACAS is an easy to use, general purpose Computer Algebra System, a program for symbolic manipulation of mathematical expressions. It uses its own programming language designed for symbolic as well as arbitrary-precision numerical computations. A software application for Linux and Unix systems. Licensed under the GPL.

Mathomatic
From the website "Mathomatic is a portable Computer Algebra System (CAS) written entirely in the C programming language. It is free and open source software, published under the GNU Lesser General Public License." There are versions available for Linux, Windows, OS/X and a web-based client.

The Yorick programming language
Includes scientific visualization functions. ... Yorick is fast enough ... for state-of-the-art physics simulation.
Download packages or source code for Linux: http://www.ibiblio.org/pub/Linux/apps/math/matrix/