MapServer is one of, if not THE granddaddy of the open source GIS tools. Developed for NASA by the University of Minnesota in the 90s, MapServer was adopted by the OSGeo and development continues today. MapServer is cross-platform and MIT-licensed, so it's used by tons of organizations and projects.
MapServer serves maps - it can generate and serve maps based on tons of input, scripted associations, custom development, base layers, output technologies, etc. It is not a tool for geospatial queries and analysis, however.