Slack - A messaging app for teams who see through the Earth!
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.
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Chatib - Chatib is an online chat room where you can meet and chat with new people, make them friends, or even data them online.
Mattermost - Mattermost is an open source alternative to Slack.
Kandan - Open Source private chat server. Your own private Chat server in minutes, plus additional features.