Over at the Zettelkasten Forum, we are using a PHP forum software called Vanilla Forum”. (Vanilla is free and open source if you host it yourself, but the creators also offer managed hosting with some additional features. I like it, it works well, and is pretty well-worn by communities all over the web.)
We’re using a Vanilla Forum for the Zettelkasten Method and ME Improved. It’s a PHP-based forum, and I like how modular it’s built. Writing plugins and themes isn’t a pain, and that’s something in my book already. It also allows us to power the comments with the forum. When you are the first to write a comment, the built-in “System” user creates a discussion with the blog post’s title and a short excerpt and your comment is added.