At Vistaprint (www.vistaprint.com), we have an extensive internal wiki, and over the last six years, my team has built 50+ custom MediaWiki extensions. I'm happy to announce that we are starting to release these extensions as open source. They range from very small (see below) to quite powerful & unique.
To help us get familiar with Wikimedia's Git/Gerrit process, we're beginning with a very small but useful extension: CategoryTagSorter. It simply alphabetizes the category links displayed in articles. (Normally they appear in the same order written as in the wikitext.) It has been called "the simplest useful MediaWiki extension" and was written originally by Dan Barrett (User:Maiden_taiwan, also author of the O'Reilly MediaWiki book).
Have a look at the documentation at: http://www.mediawiki.org/wiki/Extension:CategoryTagSorter View the code at: https://git.wikimedia.org/log/mediawiki%2Fextensions%2FCategoryTagSorter
We are looking forward to releasing more extensions in the coming months, and hope to become more involved in the MediaWiki community in general. Questions and/or comments can be sent to me directly, or to MediawikiExtensions@vistaprint.com.
Cheers, --Daniel Renfro (User:AlephNull) and the rest of the MediaWiki development team at Vistaprint