The Tango project (see "icons" above) runs their website on a customized mediawiki. http://tango.freedesktop.org/Tango_Desktop_Project
The Blender project has a nice custom skin http://wiki.blender.org/index.php/Main_Page
I've seen some other projects that have nice mediawiki skins, but I never bothered to write them down. A quick search reveals several projects by Suse: