SOGo - SOGo is groupware server with a focus on scalability and open standards.
Baïkal - Cal and CardDAV server
Radicale - The Radicale Project is a complete CalDAV (calendar) and CardDAV (contact) server solution.
AgenDAV - AgenDAV is an Open Source CalDAV Web application.
Xandikos - Lightweight CalDAV/CardDAV server on top of Git.
Calypso - Calypso is primarily a multi-user, multi-channel reservations system with a wide range of customer and technology driven features and functionalities.