Slack - A messaging app for teams who see through the Earth!
Mattermost - Mattermost is an open source alternative to Slack.
Foxo - Health industry's most powerful secure messaging platform
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
Cisco Jabber - Cisco Jabber lets you access presence, instant messaging (IM), voice, video, voice messaging, desktop sharing, and conferencing.