AstraChat is an enterprise chat and messaging product that provides privacy and security out of the box.
Mattermost - Mattermost is an open source alternative to Slack.
Slack - A messaging app for teams who see through the Earth!
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Zulip - Chat for distributed teams. Zulip combines the immediacy of real-time chat with an email threading model. With Zulip, you can catch up on important conversations while ignoring irrelevant ones.
Flock - A faster way for your team to communicate
Rocket.Chat - Rocket.Chat is a Web Chat Server, developed in JavaScript, using the Meteor fullstack framework.