Janrain-engage-extension-for-MediaWiki is a project mainly written in PHP, it's free.
An extension for MediaWiki that allows setup of a single sign-on via Janrain Engage.
JanrainEngage (http://www.janrain.com/products/engage) is an SSO utility. Hooking it into a MediaWiki extension, it becomes possible to use Facebook, OpenID, Google, etc., to sign on in MediaWiki instances.
To get access to the information you need, you will need to sign up for a JanrainEngage API key at www.janrain.com. It's free to sign up! Just be sure to give your domain access to the key (on your dashboard go to Your Domains->manage).
To add this extension to MediaWiki, just add two lines to the bottom of LocalSettings.php:
require_once('extensions/JanrainEngage/JanrainEngage.php');
$wgJERpxApiKey = "
and copy all these files into the extensions directory in a folder called JanrainEngage. Have fun!