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.
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.
Hive - Seamless project management and collaboration for your team.
Matrix.org - Matrix is an open standard for decentralized persistent communication over IP.
Element.io - Secure messaging app with strong end-to-end encryption, advanced group chat privacy settings, secure video calls for teams, encrypted communication using Matrix open network. Riot.im is now Element.