Open Source Communication Software
Ghost
Ghost is a free and open-source headless CMS and blogging platform developed by Ghost Foundation. With Ghost you can manage content in style, the user experience provided by the software is intuitive and encourages writing. Ghost is written in JavaScript for the Node.js platform and is released under the MIT license.
Adium
Adium is a free and open source instant messaging client for macOS. Adium can connect to to common messaging services and networks such as AIM, XMPP (Jabber), ICQ, Google Talk, IRC and more.
Contact Form Builder for WordPress
Contact Form Builder is a popular plugin used to create forms for WordPress-powered sites. With Contact Form Builder for WordPress it's simple to create feature rich forms that can capture contact information and other useful form data.
Mastodon
Mastodon is a free and open source decentralized social network that is used by over 4.4 million people. At mastodon you can follow friends, discover and rally around topics, and share links, pictures, text, video, much like Twitter. The main difference with Mastodon is the decentralized nature of the software. Anyone is free to start their own Mastodon server, that automatically interconnects with all the other Mastodon servers. Since the network is community-owned and not backed by any venture capital you can be sure that your data will not be monitized and that you will have an ad-free experience.
Elgg
Elgg is an award-winning open source social networking engine that provides a robust framework on which to build all kinds of social environments, from a campus wide social network for your university, school or college or an internal collaborative platform for your organization through to a brand-building communications tool for your company and its clients.
Diaspora
Diaspora is a privacy-aware, distributed, open source social network developed by the Diaspora Foundation. The social network is built up of distributed pods, that can be self-hosted by anyone. However, to join the Dispora, you can simply chose an already hosted pod on the network. Members can share and discuss content with others, and label content with hashtags, allowing them to easily follow certain topics that they are interested in. Diaspora also supports @mentions and re-shares of posts, features which all adds up to an intricate communication and sharing system, distributed across the globe.
Movim
Movim is a distributed social platform, with web client and mobile apps, that protects your privacy, comes with a set of awesome features and uses the industry-standard XMPP protocol.
HumHub
HumHub is a free and open-source social network software and framework built to give you the tools to make communication and collaboration easy and successful. HumHub is designed to be lightweight, powerful and comes with an user-friendly interface that is easy to use both for users of the social network and administrators.
Ripcord
Ripcord is a desktop chat client for group-centric services like Slack and Discord. It provides a traditional compact desktop interface designed for power users. It has a small resource footprint, responds quickly to input, and gets out of your way. With Ripcord, you can connect with friends, family and co-workers.
Jami
Jami is a free, open-source. universal distributed communication platform which preserves the user's privacy and freedoms. Jami supports a variety of video input options, including mutliple cameras and image and video files, and the selection of audio inputs and outputs with high quality audio and video codecs.