BuddyPress

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bbPress

A next generation user profile and social networking plugin for WordPress. PeepSo allows you to quickly and effortlessly create a social network and online community with you WordPress-powered site. The foundation of PeepSo is 100% free and gives you the basics to create a functional community. For more advanced features, there is a extensions ecosystem for both free and premium plugins that can be used to add more functionality and features to your site.

MemberPress is a premium WordPress plugin that allows you to instantly start charging your users for access to your content. The MemberPress plugin integrates with the most common payment provides and third-party services.

Ultimate Member is a WordPress plugin offers a wide range of features including user profiles, member directories, user registration and login, user role editing, content restriction.

Disqus is a Comment-as-a-Service provider that makes it easy to add comments to any web property. It's one of the most popular comment plugins on the market today. With Disqus, publisher can bring their content to life, fuel colorful discussions, efficiently moderate comments, and build engaged communities. The plugin takes only a matter of minutes to setup and is as easy as installing a WordPress plugin or adding a code snippet to your website.

A free and open-source modern forum software written in PHP. With Flarum you can eaisly setup your own discussion froum and host it online.

vBulletin is a powerful commercial forum software written in PHP. The software is built to be easily extended through plguins and customized with themes. vBulletin is used to power many of the largest forum, communities and social site across the web. Over 100,000 sites are currenty powered by vBulletin.