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Mattermost is an open source, self-hosted Slack alternative.

Matrix.org is a growing ecosystem for open, decentralised chat, VoIP and IoT communication.

Rocket.Chat is a Web Chat Server, developed in JavaScript, using the Meteor fullstack framework.

HipChat is an instant messaging service designed for companies and teams. It has persistent chat rooms, one-to-one chat, searchable chat history, file sharing via drag & drop and clipboard, inline image previews, an API, chat over SMS, and more.

Self-hosted chat for small teams.

What does it do:

dialog app is built and designed for corporate communication on all types of mobile devices - tablet, mobile phones etc.

qiscus is a messaging system designed to mimic working environments by allowing users to structure their teams or projects in room-based discussion system thereby removing the need for other communication tools such as emails, messaging or video chat.

TribeScale is a startup that aims to enhance teamwork in companies through the improvement of communication, coordination, and the pace on the basis of targets set. It is designed by taking as a reference the methodologies of the most innovative teams in the world: (Apple, Tesla, Google, Virgin...).Companies that use TribeScale have said that it has ended with 80 percent of working meetings. We are a platform for communication and management of teams very advanced and action-oriented, with functionalities such as creating and assigning tasks, sending surveys, decision-making, and automation of processes. »

Ryver provides all of your team communications in one place: chats, posts, and files. Invite as many team members and guests as you need.

An IRC-like internet chatroom hosted on ZeroNet, a distributed network. 0rc is a fully distributed website, and messages are shared through peers using the ZeroNet stack. In addition, making a new chatroom website is a simple as downloading the files and cloning the site.