 <p><a href="http://www.mediawiki.org" rel="nofollow">Mediawiki</a> which is used for Wikipedia, does support private wikis provided you configure it properly. Please check the following links to get an idea about it- <a href="http://www.mediawiki.org/wiki/Manual%3aPreventing_access" rel="nofollow">http://www.mediawiki.org/wiki/Manual:Preventing_access</a> <a href="http://mythopoeic.org/mediawiki-private/" rel="nofollow">http://mythopoeic.org/mediawiki-private/</a> N.B: I haven't tried it, but it should work.</p>