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 = "<yourapikey>";

and copy all these files into the extensions directory in a folder called
JanrainEngage.  Have fun!