Open Source GIS Software

Actually there is a ton of free GIS software out there, but I've found QGIS to be the best of the bunch:

"Quantum GIS (QGIS) is a powerful and user friendly Open Source Geographic Information System (GIS) that runs on Linux, Unix, Mac OSX, Windows and Android. QGIS supports vector, raster, and database formats. QGIS is licensed under the GNU Public License. "

Features:

• Direct viewing of vector and raster data in different formats and projections. Supported formats include: 
• Mapping and interactive exploration of spatial data. Tools include: 
• Create, edit and export spatial data 
• Perform spatial analysis,  
• Publish your map on the internet using QGIS Server or the "Export to Mapfile" capability (requires UMN MapServer) 
• Adapt QGIS to your needs through the extensible plugin architecture.