Discourse - Discourse is an open source discussion platform built for the next decade of the Internet.
Mobilize - Mobilize is a scalable messaging app built for large groups.
Flatboard - A Simple, Lightweight, Modern and Fast flat-file forum system written in PHP and Json.
NationBuilder - The world's first platform for leaders, handcrafted from scratch to help you grow your community and lead them to action.
ZeroTalk - Forum(s) for the Distributed network Zeronet
FUEL CYCLE by Passenger - Think Passenger builds online platforms that allow brands to connect, create and communicate with key stakeholders.