Dialog Messenger - handy and feature-rich enterprise multi-device messenger available for server or cloud – Slack-like, but not Slack-limited
Ripcord - A desktop chat client for Discord and Slack
Ziggs - Smoothly Share Content Between Devices!
Weavy - The complete white-label framework for in-app team messaging and collaboration.
LeggoChat - A channel or group based chat that is easily set up for casual, short, or serious discussions.
Crugo - Crugo is a simple group collaboration platform for your team