List

Most widely used open source applications:

GRASS – Originally developed by the U.S. Army Corps of Engineers, open source: a complete GIS
MapServer – Web-based mapping server, developed by the University of Minnesota.

Other open source GIS applications:

SAGA GIS – – System for Automated Geoscientific Analyses- a hybrid GIS software. SAGA has a unique Application Programming Interface (API)and a fast growing set of geoscientifc methods, bundled in exchangeable Module Libraries.
Quantum GIS – QGIS is a user friendly Open Source GIS that runs on Linux, Unix, Mac OS X, and Windows.
MapWindow GIS – Free, open source GIS desktop application and programming component.
ILWIS – ILWIS (Integrated Land and Water Information System) integrates image, vector and thematic data.
uDig
gvSIG – Open source GIS written in Java.
Chameleon – Environments for building applications with MapServer.
GeoNetwork opensource – A catalog application to manage spatially referenced resources
GeoTools – Open source GIS toolkit written in Java, using Open Geospatial Consortium specifications.
OpenMap
JUMP GIS – Java Unified Mapping Platform. (See end of page for JUMP derivative projects, like Kosmo and OpenJUMP)
MapGuide Open Source – Web-based mapping server.
OpenLayers – open source AJAX library for accessing geographic data layers of all kinds, originally developed and sponsored by MetaCarta
PostGIS – Spatial extensions for the open source PostgreSQL database, allowing geospatial queries.
TerraView – GIS desktop that handles vector and raster data stored in a relational or geo-relational database.
Xastir A special purpose Open Source GIS application used as an APRS client by Amateur radio operators, written in C for X Window environments."

Open Source GIS Software