MapServer 6.0 has been officially released. It brings a number of improvements and enhancements, outlined in the official release announcement.
Notable improvements include the implemntation of single-pass queries, and an overhaul to the rendering API and espression parser. Some of the new features that look pertty interesting include OpenGL rendering support, KML output, combining mupltiple layers, and clustering points.
The changes do require careful migration, so be sure to refer to the 6.0 migration guide.
[Editor's note: MapServer, in addition to being open source, is one of the oldest, most stable and efficient existing web mapping server, having won most FOSS4G benchmark shootouts so far, well, at least in 2009 and 2010 ;-)]