This is a fork of NTBB (NovaTech Bulletin Board), which ran the Showdown forums in the past. (We currently don't have our own forums, opting to share Smogon.)
I don't know how easy it would be to get it up and running, but it would probably involve a decent amount of effort. I'd be willing to help if you plan to contribute documentation or something. Open an issue! Or a PR, if you manage to get it working!
It's historically interesting, and a lot of the neat UI ideas still aren't used in modern forums, but it's probably too outdated for serious use.
Probably the most usable part is js/ntbb-richtexteditor.js. It's much simpler than modern contenteditable rich text editors, and writing your own is still a huge mess. It does have a jQuery dependency, though.
MIT