UseModWiki is the WikiEngine, used to host the MeatballWiki Forum (see AboutThisSite). It is written in Perl, and is available under the GPL. UseModWiki is loosely based on Wiki:AtisWiki and Wiki:CvWiki.
The usemod perl script is about 5400 lines long. The last official release is 1.2.1.
EricScheid has apparently done interesting things too.
I was reading a page here on meatball wiki a couple of weeks back, where there was an effort to develop of set of standard default pages to be used as a base starting point for any new usemod wiki installation. At least I think it was on this wiki, but now I can't find it anywhere. Anyone know the page I'm talking about? -- HarryWood -- 20th Feb 2005