Elgg

Elgg empowers individuals, groups and institutions to Create their own fully-featured social environment. Elgg, started in 2004, is an open source social engine which powers all kinds of social environments - from education to business and sports such as rugby. If you are looking to create your own social application or want to build and run a site for your organisation, Elgg is a great choice as it provides the Building Blocks you will need. Elgg v1.6.1 is mainly a security and bug fix release, you can find a change log in the root of the package.