PunBB
From The Forum Wiki
PunBB was founded in 2003 to be a lightweight, fast forum eschewing some common features to be add-ons (e.g. polls were not a core feature).
PunBB's lead developer left the project in 2008, for which principle development was forked into FluxBB by a new team.
There is a small but dedicated community still maintaining PunBB over at https://punbb.informer.com/
GitHub repo: https://github.com/punbb/punbb
