iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.
Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
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.
Mighty Networks - Mighty Networks enables entrepreneurs, organizations, and companies to create and grow a community-powered brand.