SpeedCrunch

SpeedCrunch is a fast, high precision and powerful desktop calculator.
Available for Linux, Microsoft Windows, and Mac OS X.

Texmaker

Texmaker is a free LaTeX editor, that integrates many tools needed to develop documents with LaTeX, in just one application.

Graph Sketcher

Graph Sketcher lets you make elegant and precise graphs in seconds, simply by sketching what you want.

Gnuplot

Gnuplot is a portable command-line driven interactive data and function plotting utility for UNIX, IBM OS/2, MS Windows, DOS, Macintosh, VMS, Atari and many other platforms. The software is copyrighted but freely distributed (i.e., you don't have to pay for it). It was originally intended as to allow scientists and students to visualize mathematical functions and data. It does this job pretty well, but has grown to support many non-interactive uses, including web scripting and integration as a plotting engine for third-party applications like Octave. Gnuplot has been supported and under development since 1986.

Gnofract 4D

Gnofract 4D is a free, open source program which allows anyone to create beautiful images called fractals. The images are automatically created by the computer based on mathematical principles. These include the Mandelbrot and Julia sets and many more. You don't need to do any math: you can explore a universe of images just using a mouse. It runs on Unix-based systems such as Linux and FreeBSD.

Magic Number Machine

Magic Number Machine is a free, full-featured, graphically laid out, high-precision, scientific calculator for Mac OS X 10.4 and greater. Full source-code is included with the distribution.

Ideal if you need to enter large expressions or have accurate precision. "Data" drawers allow an easy way to generate statistical data, linear regression and Gaussian elimination. The extensive support of complex numbers and hexadecimal numbers is also a significant benefit for anyone who has to work with this type of data.

Requires Mac OS X 10.4 or greater. Native Intel Mac support (universal binary). Source code requires XCode 2.2 or greater.