maptools.org
a resource for users and developers in the open source mapping community, and a home to many open source projects.

FreeGIS.org
FreeGIS.org maintains an extensive listing of free software, data and documents with additional infos and search opportunities.

OpenSourceGIS.org
OpenSourceGIS.org attempts to build a complete index of Open Source / Free GIS related software projects.

OSGeo.org
The Open Source Geospatial Foundation's goal is to support and build the highest quality open source geospatial software.

RemoteSensing.org
RemoteSensing.org hosts and supports various open source software projects related to remote sensing, GIS, mapping and advanced image processing.