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.
Baïkal - Cal and CardDAV server
Roundcube - Web-based IMAP email client
Radicale - The Radicale Project is a complete CalDAV (calendar) and CardDAV (contact) server solution.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
SOGo - SOGo is groupware server with a focus on scalability and open standards.