Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
SOGo - SOGo is groupware server with a focus on scalability and open standards.
Roundcube - Web-based IMAP email client
Baïkal - Cal and CardDAV server
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
Radicale - The Radicale Project is a complete CalDAV (calendar) and CardDAV (contact) server solution.