https://www.mediawiki.org/wiki/Extension:Blackout
This is NOT the code that the Wikimedia Foundation is using for the SOPA/PIPA blackout. (We'll be putting up a technical guide sometime in the next 6 hours explaining how we're implementing this, mentioning that you can't edit via the API, etc.) This is an extension developed by Gregory Varnum and John Du Hart to help other MediaWiki sites black out. The code's at:
http://svn.wikimedia.org/viewvc/mediawiki/trunk/extensions/Blackout/
That page also links to other resources and methods for anti-SOPA JavaScript and so on.