Discourse - Discourse is an open source discussion platform built for the next decade of the Internet.
Flarum - Flarum is the next-generation forum software that makes online discussion fun. It's simple, fast, and free.
Publish0x - Publish0x is a crypto agnostic, Medium.
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!
Flatboard - A Simple, Lightweight, Modern and Fast flat-file forum system written in PHP and Json.
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.