Slack - A messaging app for teams who see through the Earth!
Dialog Messenger - handy and feature-rich enterprise multi-device messenger available for server or cloud – Slack-like, but not Slack-limited
Mattermost - Mattermost is an open source alternative to Slack.
Ripcord - A desktop chat client for Discord and Slack
Skype - Stay in touch with your family and friends for free on Skype. Download Skype today to chat and call on desktop and mobile.