With costs for academic mathematics software approaching $1,000 for a single license, it doesn't take a numbers whiz to realize that a free, open source option is probably a better alternative. Sage is an entire mathematics system licensed under the GPL and available as a free download for Windows (running VMWare), Mac OS X, and Linux.

Sage covers everything from basic algebra all the way up to complex math processes like graph theory and exact linear algebra. It's designed for both research and studying, and is also plenty robust for classroom settings. In fact, the Web site has several resources for teaching with Sage, complete with lesson plans and worksheets.