Quantum GIS is an open source Geographic Information System in “C” language that supports most geospatial vector and raster file types and database formats. Unlike many other open source GIS programs, QGIS is available for a number of operating systems, including both Windows and Mac.
QGIS can be downloaded from qgis.org. The basic functions are there; however one doesn't see all the features of QGIS without trying its hundreds of plugins from the QGIS Python Plugins repository. The degree of difficulty with QGIS is not terrible as long as you follow the steps carefully, but training and tutorial before use is recommended.