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Discourse is an open source discussion platform built for the next decade of the Internet. A modern forum, everything's been redesigned to be easy.

Friendica is a distributed social network application which runs on commodity hosting platforms - PHP/MySQL/Apache. (If you can install Drupal or Wordpress you can probably install Friendica)

Flarum is free, open-source forum software with a focus on simplicity. You can use Flarum to easily set up a discussion forum for your website. Flarum is developed openly on GitHub.

Since its creation in 2000, phpBB™ has become the most widely used Open Source forum solution. Like its predecessors, phpBB™ 3.2 “Rhea” has an easy to use administration panel and a user friendly installation process, which allows you to have a forum set up in minutes.

HumHub is a free social network software and framework built to give you the tools to make teamwork easy and successful.

Social networking in a box. Build a social network for your company, school, sports team or niche community all based on the power and flexibility of WordPress. Install BuddyPress directly through your WordPress dashboard plugins page, or download it manually from the WordPress.org plugins repository.

Open source PHP social networking software with Facebook-style newsfeed. Free plugins, themes, easy installation and auto-updates.

Stay Connected. Be with Friends in Your World. Talk to Anyone in the World.

Elgg is a leading open source social networking engine which can be used to power your social network.

Noosfero is a web platform for social networks with blog, e-Porfolios, RSS, thematic discussion and events agenda in the same system.Get to know, use it, participate and contribute to this free software project.

Social Network For Joomla to grow your communities with the least time.

Socibd is global platform that lets people easily build a social networking site in minutes.