Slack - A messaging app for teams who see through the Earth!
Yac - Take your time back from Zoom & Slack
9mCollab - Collaborate (efficiently) Conclude (fast) Contribute (in the knowledge base)
Mattermost - Mattermost is an open source alternative to Slack.
LinkedIn Voice Messaging - Communicate when you want, how you want
Quip Chat Rooms - Real-time chat with your team