During my research of the current state of the open source GIS community and the viability of the surrounding software, I discovered Community Mapbuilder. It is a powerful AJAX-enabled geographic mapping client that runs in a web browser and supports the Open Geospatial consortium (OGC) standards.
It renders maps from Web Map Services (WMS), so we can integrate it easily with UMN MapServer configured as a WMS server. MapServer is an open source development environment for building spatially-enabled web mapping applications and services.
Using the latest stable Community Mapbuilder 1.0.1 release, I managed to get it running against my MapServer installation on Mac OS X.
btw, to check if your MapServer is built as WMS server, check what mapserv -v reports:
$ ./mapserv40 -v
MapServer version 4.10.3 OUTPUT=GIF OUTPUT=PNG OUTPUT=JPEG OUTPUT=WBMP OUTPUT=SVG SUPPORTS=PROJ SUPPORTS=FREETYPE SUPPORTS=WMS_SERVER SUPPORTS=WMS_CLIENT SUPPORTS=WFS_SERVER SUPPORTS=WFS_CLIENT SUPPORTS=WCS_SERVER INPUT=EPPL7 INPUT=OGR INPUT=GDAL INPUT=SHAPEFILE