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.
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
WhatsApp - WhatsApp Messenger: More than 1 billion people in over 180 countries use WhatsApp to stay in touch with friends and family, anytime and anywhere.
Slack - A messaging app for teams who see through the Earth!
Facebook Messenger - Facebook Messenger is a faster way to message.
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.