Flarum

Teams that want a forum they own outright and have somebody who can run a PHP application. Flarum is MIT licensed, installs on your own server, and has no company behind it selling a hosted version, a support contract or an enterprise edition.

Nothing is metered. No member cap, no staff seats, no storage limit, no transaction fee, no API quota, because there is no vendor between the forum and its database. What it needs instead is infrastructure: Apache or Nginx, PHP 8.3 or newer with eleven extensions, and MySQL, MariaDB, SQLite or PostgreSQL, plus command-line access and Composer to manage extensions. The core is deliberately minimal and most shipped features are themselves extensions, with hundreds more from the community.

The things you buy elsewhere are the things missing here. Nobody to call, no uptime commitment, no SOC 2 or ISO certificate, because certification attaches to whoever operates the service and that is now you. Version 2.0 has been in release candidate for years and the docs still warn it "isn't the final stable release yet". The bundled extension manager installs arbitrary Composer packages, and the project's own warning is to install it only if every forum admin is trusted with that.

Discourse is the other open-source option and the better-supported one, with a hosted plan if you want the vendor back. Circle and Bettermode are what the same thing costs hosted, at $89 and $399 a month. Filed under community forum software.