OddMuse: there is an extension converting LaTeX into images [1]
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.