Courtesy of Shane Bradt, of the UNH Cooperative Extension, here is a matrix comparing the functionalities of different GIS software that are free.
The comparison also includes whether there are MAC versions of the software.
From the UNH Cooperative Extension course "GIS on Pennies a Day," this is a list of freely available, open source GIS software. You can download and use the software.
QGIS and DIVA-GIS are more widely used than the others. Both have MAC versions available for downloading.
These are other open source GIS software available via the web.