I have maintained a wiki plug-in for the Elgg social networking system since January 2006.  It has gone through a number of chanages since then, and is now in its 7th major version.  It runs about 10,000 lines of code, which translates out to a pretty good-sized book.  We've been using this for several years at Claremont Graduate University, and I have a number of Publications.  To download the plugin, please go to my files.

Below is a demonstration of the latest version.