SAGE Mathematics Software
The SAGE Mathematics Software project, which is an interdisciplinary software development project at UW, involving people in math, computer science, applied math, at Microsoft Research, and elsewhere. SAGE is freely available for Windows, OS X, and Linux, and has -- or is working towards having -- comparable functionality to Maple/Mathematica/Matlab.
SAGE is particularly valuable for UW people who need technical support because there are 9 currently active SAGE developers on campus.
SAGE can be downloaded from the SAGE Web site.