Macaulay is freely downloadable with source, and is available for Macintosh, Linux, and Windows.
Macaulay2 is a rewrite of Macaulay and is published under the GPL.
Macaulay: " class="external">http://www.math.columbia.edu/~bayer/Macaulay/
Macaulay2: http://www.math.uiuc.edu/Macaulay2/External Links