Element.io - Secure messaging app with strong end-to-end encryption, advanced group chat privacy settings, secure video calls for teams, encrypted communication using Matrix open network. Riot.im is now Element.
Stork.ai - AI-Based Collaboration for Hybrid Teams. Rich media messaging, advanced video and voice conferencing, auto-transcribing and screen sharing. Serendipitous meetings at the water-cooler with your human and AI teammates.
ChatSecure - ChatSecure is a free and open source messaging app that features OTR encryption over XMPP.
Ring4 Meeting - Dead simple video conferencing - free, in browser
Keybase Chat - End-to-end encrypted chat built into Keybase
Suttna - Keep every team member in the loop