GRASS GIS, commonly referred to as GRASS (Geographic Resources Analysis Support System), is a free and open source Geographic Information System (GIS) software suite used for geospatial data management and analysis, image processing, graphics and maps production, spatial modeling, and visualization. GRASS GIS is currently used in academic and commercial settings around the world, as well as by many governmental agencies and environmental consulting companies. It is a founding member of the Open Source Geospatial Foundation (OSGeo).
The current version 7.0.3 is an LTS (Long Term Release) and its for all OS’s (Linux, Windows, Mac OS). In the official site you can find everything (documentation, tutorial & courses etc) about GRASS.