Grass

Geographic Resources Analysis Support System,  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.

From: GRASS developer community

Platform: All platforms (Windows requires the Cygwin X windows GUI)

Cost: Free - open source

Fields