The best open source alternative to Mapserver is OpenMapTiles, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to Mapserver and ten of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Mapserver are PostGIS (Free, Open Source), GRASS GIS (Free, Open Source), GeoServer (Free, Open Source) and Map3 (Free, Open Source).
GRASS GIS (Geographic Resources Analysis Support System) is an open source, free software (FOSS) Geographical Information System (GIS) with raster, topological, vector, image processing, and visualization functionality.
The reference implementation of the Open Geospatial Consortiums WFS and WMS services, GeoServer is a full featured open source software server written in Java that allows users to query, visualize, combine, edit and publish spatial data over the web.
Map3 is a decentralised map network that is impartial, secure and extremely robust. It is free from control, free to participate, free base map. It is built to be governed by the Map3 community where individuals and organisations can host Map3 nodes.
Mapnik is a Free Toolkit for developing mapping applications. Above all Mapnik is about making beautiful maps. It is easily extensible and suitable for both desktop and web development. - C++/Python GIS toolkit
uDig is a GIS software program produced by a community led by Canadian-based consulting company Refractions Research. It is based on the Eclipse platform and features full layered open source GIS. It is written in Java and released under GNU Lesser General Public License.