Source: [itsfoss](https://itsfoss.com/open-source-forum-software/)

1. [Discourse](https://github.com/discourse) (modern and popular) — Ruby
2. [Talkyard](https://github.com/debiki/talkyard) (inspired by Discourse and StackOverflow) — NodeJS
3. Forem (Unique approach for a community platform)
4. NodeBB (Modern and full of features)
5. Vanilla Forums (enterprise focused)
6. bbPress (from WordPress)
7. phpBB (classic forum software)
8. Simple Machines Forum (another classic)
9. FluxBB (old school)
10. MyBB (less popular but worth a look)
11. Flarum
12. Bonus: Lemmy (More like a Reddit alternative but a decent option)