Mattermost

Discuz! X is an internet forum software written in PHP and supports MySQL and PostgreSQL databases.

PeerBoard is a plug-and-play community solution, which helps groups, clubs, startups, marketplaces and businesses create discussion forums.

A commercial forum software known for its extensive feature set and polished user interface. IPB offers a suite of integrated applications including forums, blogs, galleries, and downloads. It provides advanced moderation tools, customizable member groups, and a powerful theme system. IPB's strength lies in its all-in-one community platform approach, making it suitable for businesses or organizations looking to create a comprehensive online community beyond just forums.

Reddit is a massive social news aggregation, web content rating, and discussion platform. It allows users to submit content and participate in discussions organized into user-created "subreddits". Reddit's unique voting system, where users can upvote or downvote content, helps surface popular content. Its API and extensive moderation tools make it a powerful platform for building large-scale online communities around specific topics or interests.

MyBB is a free and open-source forum software written in PHP.