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.
As access is restricted, due to the nature of the wiki you need to search from information about Intellipedia, from CIA press release and off course the Intellipedia Wikipedia entry. I would recommend reading both and looking for more information of the web on how Intellipedia became so successful. Because if you every need a good example of a cross agency wiki working, Intellipedia is it.