FreeBSD forums, using XenForo which doesn't seem to be open source: https://forums.freebsd.org/

Arch Linux forums, using FluxBB: https://bbs.archlinux.org/

Free Pascal forums, using Simple Machines Forum: https://forum.lazarus.freepascal.org/

Python forums, using Discourse: https://discuss.python.org/

Nim forums, using their own nimforum software written in Nim: https://forum.nim-lang.org/