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.
The free items refer only to the actual software. Other resources such as eBooks may require a Dartmouth sign-on or you can use an alternative method for access.
These are other open source GIS software available via the web.