QGIS - QGIS is a desktop geographic information system, or GIS.
PostGIS - Open source spatial database
GRASS GIS - GRASS GIS is a free Geographic Information System (GIS) software used for geospatial data management and analysis, image processing, graphics/maps production, spatial modeling, and visualization.
H2GIS - A spatial extension of the H2 database. Compliant, Solid, Tested.
gvSIG - gvSIG Mini Maps allows you to browse and navigate with GPS a full set of free on/offline maps from...
SAGA GIS - SAGA - System for Automated Geoscientific Analyses - is a Geographic Information System (GIS)...