Quoted from: http://wwwamb.bologna.enea.it/forgrass/

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GRASS GIS (Geographic Resources Analysis Support System) is an open source, Free Software Geographical Information System (GIS) with raster, topological vector, image processing, and graphics production functionality that operates on various platforms through a graphical user interface and shell in X-Windows. It is released under GNU General Public License ( GPL ).

After download you can INSTALL AND WORK AFTER FEW MINUTES.
Now you can install all files with a simple installer and you can run Grass 6.0 in Mac OS X environment from an icon in Finder and the menu up. Double clic on file and you enter in the power of Grass.

PRODUCTS

• GRASS 6 - this version includes all: libraries, grass 6.0.0 with Aqua/X11 interface, visual tutorial, docs, links and Extra software.
• GRASS 6 CVS - it only includes last grass 6.1 cvs application (development version). GRASS 6 is required.
• GRASS 5 - it only includes grass 5.4.0 application. GRASS 6 is required.

FIVE STEPS FOR WORKING

1. Download GRASS 6 (the longer step)
2. Read the INSTALL file inside
3. Install Grass Libraries (all libraries for Grass)
4. Drag Grass folder into Applications folder (includes Grass 6.0.0 and all docs)
5. Double clic on grass60 icon and enter in Grass