BuddyPress, the suite of  WordPress plugins and themes designed to work with WPMU, the multi-user version of WordPress, is now available in a version for regular WordPress sites.

BuddyPress adds features like a user profile, ability to add friends, instant messaging, groups, activity streams and more to your site. It allows your members to interact and connect.

You can get all the specifics on how to add BuddyPress to a single user WordPress install on the MPMU.org site:

http://wpmu.org/buddypress-now-supports-wordpress-single-user-no-wpmu-required/