Be sure to check MapServer .
Both MapServer and GeoServer support OGC-compliant WMS requests, so they are mostly interchangeable if you use something like OpenLayers.
MapServer received a convenient installer that allows you to install every aspect that may be required to run your server and web application. It runs on Apache, which I find a big advantage over GeoServer, which runs on Jetty (this is all-java).
However, now I am using GeoServer. It's really easy to administer stuff through its web interface, and overall it just feels pretty polished. Maybe because I have been using this for a while, but I'm just doing it fast.
Wouter van nifterick
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