Intellipedia
aka Spook 2.0
A wiki that covers a over a dozen different agency, with over 35,000 articles, 200,000 pages, 37,000 users, 100 new articles and 4,000 edits a day is pretty impressive. Especially when you find out the agencies involved are intelligence agencies who had reputation for being less than willing to share their information. It has to be said this is a very successful wiki implementation.