http://wordpress.org/extend/plugins/wiki-append/
WordPress plugin that appends a wikipedia (or other mediawiki site) entry to the bottom of a wp post / page. Uses ?action=render on the end of the url to get just the HTML for the article, not the site. Useful as a resource.
http://moinmo.in/
Wiki written in python as used by Ubuntu, FreeBSD, Debian and others. Uses flat file storage by default. Lovely.
http://corp.kaltura.com/
Kaltura is an open source video collaboration tool. It integrates with MediaWiki to allow wiki-like editing of videos. The software is open source and hosted on sourceforge.
http://www.wikia.com/wiki/Wikia
Wikia allows you to create free (I think) MediaWiki installs. They currently host over 4'700 apparently.
http://wikkawiki.org/HomePage
Wikka is a very simple PHP/ MySQL wiki. It has very basic functionality, without the advanced features of something like MediaWiki, but it's extremely easy to use.
http://www.mediawiki.org/wiki/Manual:Upgrading
MediaWiki upgrading instructions, should apply to any version.
http://www.openwebanalytics.com/
An open source web analytics framework with in-built integration for WordPress and MediaWiki.