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.
GRASS 6 is packaged for Debian. It is released under GNU General Public License (GPL).
GRASS has been a Debian package for some weeks now. The maintainer is looking for 'vicitims' too. It is in unstable.