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.
Skype - Stay in touch with your family and friends for free on Skype. Download Skype today to chat and call on desktop and mobile.
Microsoft Teams - Microsoft Teams provides the enterprise-level security, compliance and management features you expect from Office 365, including broad support for compliance standards, and eDiscovery and legal hold for channels, chats, and files.
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.