GRASS - free and open-source geospatial processing engine
Mac homebrew tap for maintaining a stable work environment for the OSGeo.org geospatial toolset
OSGeo-Live is a self-contained bootable DVD, USB thumb drive or Virtual Machine based on LUbuntu, that allows you to try a wide variety of open source geospatial software without installing anything. It is composed entirely of FOSS, allowing it to be openly reproduced and distributed.
GRASS Tutorials Repository
GRASS GIS promotional material repository