iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
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.
Kolab Community - Kolab Groupware is a secure, scalable, reliable, flexible, full-featured collaboration suite.
Roundcube - Web-based IMAP email client
hMail Server - hMailServer is a free e-mail server for Microsoft Windows.
SOGo - SOGo is groupware server with a focus on scalability and open standards.