Open Source Forum Software
A forum is built to outlast its threads - the value compounds over years as questions get answered once and found a thousand times, which makes a platform that can paywall, delete, or simply shut down that archive a genuine risk to your community's memory. The open source options here keep every thread, account, and attachment in a database you host and can fully export, so the searchable history your members built together can't be locked behind a plan change or lost when a host disappears.
Discourse
Open-source community discussion platform with self-hosting, built-in chat, and themes
Forem
Open source software for building online communities, the platform behind DEV Community
Flarum
Simple, extensible forum software for building online communities
NodeBB
Node.js forum software with real-time discussions, web socket notifications, and Redis, MongoDB, or PostgreSQL storage
Lemmy
Decentralized link aggregator and forum for federated communities, with self-hosted servers
Vanilla
Customizable forum software for building branded community discussions and customer communities
Misago
Modern forum software in Python with Django, React.js, and OAuth login for 50+ providers
phpBB
Free open-source bulletin board written in PHP for running discussion forums
MyBB
Free and open source forum software written in PHP with MySQL, PostgreSQL, and SQLite support