BuddyPress - WordPress social networking in a box.
Yammer - Yammer is a leading enterprise social network used by more than 200,000 companies worldwide, including 85% of the Fortune 500, to get work done smarter and faster. Sign Up Today.
Elgg - Opensource social network engine
MS SharePoint - Share, organize, and discover information with Microsoft SharePoint. Learn about SharePoint Online, OneDrive for Business, and Apps for SharePoint
SocialEngine - Socialengine PHP: Elevating Your Online Community Experience.
Communifire - Enterprise Social Collaboration Software