phpBB is used by millions, making it the most used Open Source forum solution in the world. phpBB incorporates a professional-quality modular design, high security, multiple-language interface and an extensive administration panel, which allows the customization of even the most intricate features.

GNU social (formerly StatusNet) provides microblogging functionality similar to Twitter. It uses an open, distributed design that allows enterprises and individuals to install and control their own services and data while integrating with other GNU Social instances, SMS, Twitter, Facebook and XMPP IM clients.

Etherpad is a real-time collaborative editor, sort of like a web-based multiplayer Notepad that allows groups of users to simultaneously edit a text document, with the ability to display each author's text in their own color. There is also a chat box in the sidebar to allow meta communication.

phpList is the world's most popular open source email campaign manager. It's a one-way email announcement delivery system that's great for newsletters, publicity lists, notifications, and many other uses. It has a web interface lets you write and send messages, and manage phplist over the internet.