WP Symposium is an open source social networking software package. It is a suite of plugins for WordPress that turns it into a social network - although you can choose which plugins you want to use. For example, if you only want to use the Forum, you would only activate the Forum plugin.
Work on WP Symposium started in December 2010 by Simon Goodchild, and since then an active community website has been created.
As it is built on WordPress it is written using the same primary technologies, PHP and MySQL.
It consists of:
- On profile page, post messages and replies (activity)
- Member directory
- Private mail
- Chat windows
- Chat room
Further plugins available on a low-cost subscription basis:
- Alerts (via WordPress menu)
- Photo Gallery
- RSS Feed
- Share activity to Facebook
- Mobile version (forum/status) - good for SEO and accessibility
- Adds additional activity to the profile page (new forum topics, gallery uploads)
- Profile Plus adds quick hover box to avatars, and follow feature
Updates to the product are frequent - following a key philosophy of the lead developer of "release quick, fix quick". 
A full change log of releases can be found here.
Extending WP Symposium
A growing number of WordPress hooks and filters allows developers to create WPS plugins. A demonstration plugin is available via the WPS website, with more information is available on the WPS Wiki at http://www.wpswiki.com/index.php?title=Developers
Independent reviews of WP Symposium
- on WPS website 
- WPMU.org website 
- wpcandy.org 
- Mingle Communities 
- wordpressinfo.com