Open Source Communication Software
Jitsi
Jitsi is an open-source project and software toolset that allows you to easily build and deploy secure video conferencing solutions. At the heart of Jitsi are Jitsi Videobridge and Jitsi Meet, which let you have conferences on the internet, while other projects in the community enable other features such as audio, dial-in, recording, and simulcasting. Jitsi supports popular protocols such as SIP, XMPP/Jabber, ICQ/AIM, Windows Live, Yahoo!, Google Hangouts, OTR, ZRTP and more.
Freenet
Freenet is a peer-to-peer platform for censorship-resistant communication and publishing. With the Freenet client you can browse websites, post on forums, and publish files within Freenet network with strong privacy protections.
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.
BuddyCloud
Buddycloud is an open source project that helps developers add chat, video and social features to their app without being locked into a proprietary solution. With BuddyCloud it's easy to build communities that facilitates rich communication. The open-source toolset is written in Java and is designed to scale well and save you your team time, when building an online community.
Pidgin
Pidgin is a chat application which lets you log into accounts on multiple chat networks simultaneously. Pidgin is developed and released fully free and open-source and provides a plugin architecture that allow anyone to build extensions and connections to new networks and third-party services.
Mumble
Mumble is a free and open-source voice over IP application primarily designed for gamers. The application features a customizable in game overlay, application skins, conversation recording, and advanced voice activity detection. Anyone can download and use Mumble for free, but if you want your own server, you can rent a managed solution from the company behind Mumble, or host your own.
Contact Form 7
Contact Form 7 is a WordPress plugin that can be used to create and manage multiple contact forms. The plugin allow you to customize your forms and connect them to email to send confirmation emails designed with flexible and simple markup. Contact Form 7 also provides CAPTCHA and spam filtering with Akismet.
Rocket.Chat
A free, unlimited and open-source chat software. Rocket.Chat have set out to be a replacement for email, HipChat & Slack and is provided as Software-as-a-Service (SaaS) or a self-hosted community edition. Rocket.Chat is feature packed and can be used for video conferencing, file sharing, voice messages and more. The software integrates with third-party services like GitHub, GitLab, JIRA, and Confluence and provides an API that can be used for authentication and integration across these services.
K-9 Mail
K-9 Mail is an open source email client focused on making it easy to chew through large volumes of email. K-9 Mail is designed for both novice and power users and offers lots of features for everyone. It provides features like Push IMAP Support, GPG & PGP/MIME support, Per Account Notifications, Unified Inbox, White and Dark Theme, and Signatures.