Geographic Resources Analysis Support System (GRASS)
http://grass.itc.it/

Commonly referred to as GRASS, this is a Geographic Information System (GIS) used for geospatial data management and analysis, image processing, graphics/maps production, spatial modeling, and visualization. GRASS is currently used in academic and commercial settings around the world, as well as by many governmental agencies and environmental consulting companies. GRASS is official project of the Open Source Geospatial Foundation. This has been added to Research Resources Subject Tracer™ Information Blog. This has been added to World Wide Web Reference Subject Tracer™ Information Blog.