Quantum GIS (QGIS)
QGIS is an open-source alternative to ArcGIS software -- perhaps not as polished or with as many labor-saving features, but highly capable and robust. The QGIS community has created a number of plugins that further extend its capabilities.
You can load multiple data tables into QGIS for multi-layered maps, join tables on common fields, and do full-fledged geospatial visualization and analysis.
Get it: http://www.qgis.org