"GeoTools is an open source (LGPL) Java code library which provides standards compliant methods for the manipulation of geospatial data, for example to implement Geographic Information Systems (GIS). The GeoTools library implements Open Geospatial Consortium (OGC) specifications as they are developed. The capabilities of GeoTools are presented in the feature list."
"Chameleon incorporates the ability to quickly set up new applications from a common pool of widgets that can be placed in an HTML template file. These widgets provide a fixed piece of functionality, but the representation of the widget is usually highly configurable."
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.