BuddyPress

BuddyPress is an open source, social networking software package owned by Automattic since 2008. It is a plugin that can be installed on WordPress to transform it into a social network platform. BuddyPress is designed to allow schools, companies, sports teams, or other niche communities start their own social network or communication tool.

BuddyPress inherits and extends upon the integral functional elements of WordPress engine including themes, plugins, and widgets. Because it is built on WordPress, it is written using the same primary technologies, PHP and MySQL. [Wikipedia]

In general, BuddyPress helps you build any kind of community website using WordPress, with member profiles, activity streams, user groups, messaging, and more.

BuddyPress Codex