BuddyPress is our new favorite tool. It’s a massive plug-in that can be used with WordPress MU to add a virtual plethora of social networking features to your website. WordPress MU is the multi-user version of WordPress. WordPress MU is most famously used for WordPress.com where it serves tens of millions of hits on millions of blogs each day. BuddyPress adds a social networking layer to a new or existing installation of WordPress MU.