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.
Wiki written in python as used by Ubuntu, FreeBSD, Debian and others. Uses flat file storage by default. Lovely.
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.
Wikia allows you to create free (I think) MediaWiki installs. They currently host over 4'700 apparently.
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.
MediaWiki upgrading instructions, should apply to any version.
An open source web analytics framework with in-built integration for WordPress and MediaWiki.