http://yourarchives.nationalarchives.gov.uk

Your Archives, powered by MediaWiki, this site is for users (and any other interested party) of the British National Archives and is hosted by that institution. The intent is to allow users to inform their colleagues and National Archives staff about information in the records that might not necessarily be captured through the National Archives’ descriptive tools. This is likely the first use and endorsement by a national government of wiki technology for its community of users.

Quoted from
http://www.davidmattison.ca/wordpress/?p=2035

Exciting!