SOGo - SOGo is groupware server with a focus on scalability and open standards.
HEY - Email at its best, new from Basecamp.
Baïkal - Cal and CardDAV server
InstAddr - Disposable email address application.
Radicale - The Radicale Project is a complete CalDAV (calendar) and CardDAV (contact) server solution.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.