MapServer - a very popular Open Source engine for publishing spatial data and interactive mapping applications to the Web.
GRASS GIS - commonly referred to as GRASS, this is a full featured GIS used for geospatial data management and analysis, image processing, graphics/maps production, spatial modeling, and visualization.
Quantum GIS - commonly referred to as QGIS, this is a user friendly Open Source Geographic Information System (GIS) that runs on Linux, Unix, Mac OSX, and Windows. QGIS supports vector, raster, and database formats, and is often used as the visual front-end for GRASS.
ArcExplorer - ArcGIS Explorer gives you an easy way to explore, visualize, and share GIS information.