Dialog Messenger - handy and feature-rich enterprise multi-device messenger available for server or cloud – Slack-like, but not Slack-limited
Slack - A messaging app for teams who see through the Earth!
Ripcord - A desktop chat client for Discord and Slack
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Done Hui - No need to switch between multiple pieces of software to get through the workday. CHATS: Communicate freely. CALENDAR: Know your team's availability, plan meetings. No more conflicts. TO-DOs: Stay on top of all projects. FILES: All files, one spot.
Mattermost - Mattermost is an open source alternative to Slack.