Simple Machines Forum - Simple Machines offers free open source software such as SMF, the powerful and easy to use community forum written in PHP. Start interesting discussions on your website!

Bloomerang - Bloomerang is a simple donor database and fundraising software solution that helps nonprofits decrease donor attrition and increase revenue.

Raddle - A web-based forum where users submit links, pictures, posts, etc.

Kindful - Nonprofit donor database + fundraising tools all in one

Invision Community - Invision Community is a flexible, integrated set of apps for building communities on the web.

Classy - Expressive, flexible, and powerful stylesheets for native iOS apps