Discourse, Flarum,
and Lemmy, open source and reminds of HackerNews/Reddit: https://join-lemmy.org (it's quite alive over at GitHub :-) https://github.com/LemmyNet/lemmy/graphs/contributors)
And I'm developing another one, Talkyard, which reminds of HN/Reddit as well: https://www.talkyard.io (also: https://www.talkyard.io/-32/how-hacker-news-can-be-improved-...)
It seems to me that companies often want forum software, sometimes internally, sometimes for customer support — on their own domain, rather than over at Reddit.
But for consumers, maybe, like others here write, it's mostly Reddit (for English speakers)?