GRASS - Geographic Resources Analysis Support System is a general GIS supporting processing, analysis and display of raster, vector, site and imagery data, geospatial simulations and visualization.
For a general introduction, see the GRASS First Time User Page.
Overview of GRASS versions currently available:
CVS GRASS6 - continuously updated developers version - see GRASS CVS page
GRASS6.1 - under development, see GRASS6 page
GRASS6.0 - current stable release, see GRASS6 page
GRASS4.3- last non-floating point release - see GRASS4.3 release page
GRASS4.1- latest official CERL release 1991 - see GRASS4.1 release page
GRASS is an Open source sofware released under the GNU General Public Licence - it is therefore free with protection of the authors' rights. With its 1.5 million lines of C code, it is one of the largest Open source projects.