iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Mighty Networks - Mighty Networks enables entrepreneurs, organizations, and companies to create and grow a community-powered brand.
Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
Nabble - Nabble is a forum software that are embeddable and fully customizable with scripting language.
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.
Bigtent - BigTent is a web-based organization and communication portal for groups.