Geospatial Modeling Environment (GME) grew out of Hawth’s Tools, a collection of tools that provided welcome and easy functionality with an ArcGIS-interoperable toolbar.  It has always been optimal for environmental and ecological work, but its functionality can be applied to many other application areas. Now a stand-along package, with all of the original bits and more. It has software dependencies on both ArcGIS and R; see the download page for more information.

GRASS, Geographic Resources Analysis Support System. The original open source GIS package, remembered fondly – and not so fondly – for the challenges a user would experience.  But this isn’t your father’s GRASS.  New and improved, and increasingly popular world-wide, with resources and tutorials available in multiple languages.