Mobilizon - A free and federated tool to get our events off Facebook
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
mailcow - An open source mailserver suite.
DownToMeet - DownToMeet is a platform to organize, find, and attend group events.
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.