Dialog Messenger - handy and feature-rich enterprise multi-device messenger available for server or cloud – Slack-like, but not Slack-limited
Ziggs - Smoothly Share Content Between Devices!
Ripcord - A desktop chat client for Discord and Slack
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.
Protonet SOUL - Protonet SOUL is social project management and collaboration for workgroup communication, file sharing, and task management.