Implementations

A feature that allows one to use LaTeX in the source for a page on a wiki.

ZwikiClone: LatexWiki patch.
MathWiki: running its own WikiEngine (with other improvements besides just LaTeX). Forked off of WardsWiki.
UseMod: a small patch is in progress (see UseMod:WikiSuggestions/TeX)
OddMuse: there is an extension converting LaTeX into images http://www.emacswiki.org/cgi-bin/oddmuse.pl/LaTeX_Extension
PhpWiki: the latest (CVS) version supports math mode TeX via the PhpWiki:TexToPng plugin (written by JohannesGroße)
MediaWiki (WikiPedia): uses an Objective Caml module called TexVc to parse formulas (limited subset of LaTeX), pass them on to LaTeX and to convert them into PNG images or HTML text.
NooSphere: Not exactly a wiki, but the software used at [PlanetMath]. It supports an interesting set of access and editing controls and an automatic linking mechanism which distance it from traditional wikis.
The [tcl TIP rendering engine], which converts the TIP format (which is very similar to WikiSyntax) to HTML, XML, LaTeX, and others.
The Woha wiki [go to Woha Website] enables writing of Latex formula, and other features (footnotes, comments,authentication). Writtent in Perl CGI.