Free and Open-Source CalDAV and CardDAV Server.

Radicale is a small but powerful CalDAV (calendars, to-do lists) and CardDAV (contacts) server.

Radicale @ GitHub.

The open source CardDAV, CalDAV and WebDAV server.

Sabre.io @ GitHub

Open Source Groupware.
SOGo is a fully supported and trusted groupware server with a focus on scalability and open standards. SOGo is released under the GNU GPL/LGPL v2 and above.
SOGo provides a rich AJAX-based Web interface and supports multiple native clients through the use of standard protocols such as CalDAV, CardDAV and GroupDAV, as well as Microsoft ActiveSync.

Collaborative email, calendar, contacts and tasks.
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Lightweight CalDAV+CardDAV server