Bitrix24 - Bitrix24 is a free (for small businesses) social enterprise platform. It is a united work space which handles the many aspects of daily operations and tasks. Create your own social intranet in minutes!

BuddyPress - WordPress social networking in a box.

EssentialPIM - EssentialPIM is a free Personal Information Manager that keeps up with the times and lets you manage appointments, tasks, notes, contacts, password entries and email messages across multiple devices and cloud applications.

Elgg - Opensource social network engine

LinkedIn Premium - LinkedIn is a social network helping brands and professionals to connect, identify sales prospects, hire talent, and promote their business.

SocialEngine - Socialengine PHP: Elevating Your Online Community Experience.