Slack - A messaging app for teams who see through the Earth!
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
Mattermost - Mattermost is an open source alternative to Slack.
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Notion - All-in-one workspace. One tool for your whole team. Write, plan, and get organized.
Cisco Jabber - Cisco Jabber lets you access presence, instant messaging (IM), voice, video, voice messaging, desktop sharing, and conferencing.