Zimbra - Zimbra is trusted by over 500 million users to increase productivity with a complete set of collaboration tools while maintaining total control over security and privacy.
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.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Notion - All-in-one workspace. One tool for your whole team. Write, plan, and get organized.
mailcow - An open source mailserver suite.
Slack - A messaging app for teams who see through the Earth!