Flarum - Flarum is the next-generation forum software that makes online discussion fun. It's simple, fast, and free.
Invision Community - Invision Community is a flexible, integrated set of apps for building communities on the web.
XenForo - Intuitive. Social. Engaging. Fast. XenForo brings a fresh outlook to forum software.
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.
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!