Wikia (homepage) is a free ad-based wiki hosting service that enables people to find and collaborate with people who love what you love. It uses MediaWiki and a bunch of self developed MediaWiki extensions (released under GPL). Wikia was founded in late 2004 by Jimmy Wales and Angela Beesley. It currently hosts 150,000+ wikis with more than 3 million content pages and has 30+ million visitors per month which makes it a top 100 web property.
At translatewiki.net over 100 original Wikia extensions can be translated, as well as the at least 24 extensions Wikia uses from the MediaWiki source code repository.
In practice, as at February 2010, messages are committed by translatewiki.net about once a week. Wikia makes weekly releases.