QGIS is a cross-platform (Linux, Windows, Mac) open source Geographic Information system.
The QGIS project offers different applications for your use cases:
The major features include:
QGIS Browser is a simple and fast data viewer. It allows the user to explore attribute and meta data of local and online source.
QGIS Server currently offers WMS 1.3.0 and 1.1.1 implementation. Features include:
The QGIS mapserver is a FastCGI/CGI application written in C++. It works together with a webserver (Apache in most cases) invoking the FastCGI application. It uses QGIS as backend for the GIS logic and for map rendering.
Supported plattforms include Linux, Windows XP (only CGI at the moment) and MacOSX. Other Unixes possible, but not tested.
QGIS mapserver is an open source program released under the GPL license.
Installation instructions are provided on the QGIS Wiki.