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Discourse - Discourse is an open source discussion platform built for the next decade of the Internet.
XenForo - Intuitive. Social. Engaging. Fast. XenForo brings a fresh outlook to forum software.
vBulletin - Build a community website with the leading forum software, vBulletin 5 Connect, or in the cloud on vB Cloud!
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!
NodeBB - NodeBB Forum Software - A better community platform for the modern web. NodeBB is a next generation forum software that's free and easy to use.