vBulletin - Build a community website with the leading forum software, vBulletin 5 Connect, or in the cloud on vB Cloud!
NationBuilder - The world's first platform for leaders, handcrafted from scratch to help you grow your community and lead them to action.
Discourse - Discourse is an open source discussion platform built for the next decade of the Internet.
Crowded Communities - The all-in-one community platform.
XenForo - Intuitive. Social. Engaging. Fast. XenForo brings a fresh outlook to forum software.
FUEL CYCLE by Passenger - Think Passenger builds online platforms that allow brands to connect, create and communicate with key stakeholders.