BuddyPress - WordPress social networking in a box.
Bloomerang - Bloomerang is a simple donor database and fundraising software solution that helps nonprofits decrease donor attrition and increase revenue.
SocialEngine - Socialengine PHP: Elevating Your Online Community Experience.
Kindful - Nonprofit donor database + fundraising tools all in one
HumHub - HumHub is a free social network software and framework built to give you the tools to make teamwork easy and successful.
Classy - Expressive, flexible, and powerful stylesheets for native iOS apps