NationBuilder - The world's first platform for leaders, handcrafted from scratch to help you grow your community and lead them to action.
Flarum - Flarum is the next-generation forum software that makes online discussion fun. It's simple, fast, and free.
Crowded Communities - The all-in-one community platform.
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!
FUEL CYCLE by Passenger - Think Passenger builds online platforms that allow brands to connect, create and communicate with key stakeholders.
Discourse - Discourse is an open source discussion platform built for the next decade of the Internet.