QGIS - The Leading Open Source GIS (QGIS)

QGIS is a user-friendly Open Source Geographic Information System (GIS) licensed under the GNU General Public License.  It runs on Linux, Unix, Mac OSX, Windows and Android and supports numerous vector, raster, and database formats and functionalities. The program’s core functionality is extended through plugins (e.g. for spatial query, topology checking, zonal statistics, etc.). The Python programming language is available in QGIS to automate tasks. QGIS can perform spatial data analysis on spatial databases and other OGR- supported formats. QGIS currently offers vector analysis, sampling, geoprocessing, geometry and database management tools. QGIS also uses the GDAL library to read and write raster data formats, including ArcInfo Binary Grid, ArcInfo ASCII Grid, GeoTIFF, ERDAS IMAGINE, and many more. QGIS functionality is well documented through a User Guide and  Training Manual and supported by an active user and developer community .

QGIS is a useful free GIS software for non-specialists who want to be able to visualize GIS data files and eventually perform simple analyses, but has, as all GIS software, a  significant learning curve.