Open Source Geospatial Software

The essentials:
GRASS GIS (Geographic Resources Analysis Support System)
Quantum GIS
Geospatial Data Abstraction Library (GDAL)
Simple Feature Library (OGR)
PostGIS
Mapserver
Geocoder US

Collections of links to open source geospatial projects:
Free GIS Project
Open source GIS list
Remotesensing.org
Map Tools

Selected goodies on GIS:
GIS on a shared hosting environment - good instructions on how to install GIS software on a shared hosting server - from Aaron Racicot's Blog "REPROJECTED".