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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.

Simple Machines Forum or SMF in short is a free and open-source forum software. SMF can be used to build your own online community that you self-host on a host of your choice. Simple Machines Forum comes packed with features for customization, extension and management of your community. The template engine puts you in control of the design of your community. The plugin architecture makes it easy to extend and provide further feautres though third-party built packages. 

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.

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.

Discourse is an open source internet forum and mailing list software. The application is written with Ember.js and Ruby on Rails, and can be self-hosted in the cloud or on local hardware for free. Discourse also provides managed hosting starting at $100/month, so that you can focus on building a community rather than installing software and managing servers.

LayerBB is a small yet feature rich, free and open-source community software written in PHP. LayerBB enables you to quickly and easily start your own community while providing great flexibility and customisability.