NodeBB

NodeBB is forum software in the old sense, with categories and threads, built on Node.js and released under the GPL. You can run it on your own server for nothing, or pay NodeBB to host it and keep the plugins and backups somebody else's problem. Hosted plans are sized by pageviews rather than by seats, so a busy community with three moderators costs the same as a quiet one with thirty.

Self-hosting is free under GPLv3 and is a genuine option, not a crippled edition. Hosted plans start at $20 a month for 50,000 pageviews and 5GB of storage, rising through $100 for half a million pageviews, then $250 and $750. Every tier carries unlimited staff users, SSL, a custom domain and custom plugins, so the ladder is about traffic rather than features. All prices are stated in USD, the trial runs fourteen days and takes a card, and special pricing is offered to non-profits, startups and open source projects.

A community whose value is in the archive: questions asked once and read for years, indexed by search engines you do not control. If your members expect a feed and notifications on their phone, they will find this quiet and dated, and Circle or Mighty Networks fits better.