Open source push technology.

Z-Push is an open-source application to synchronize ActiveSync compatible devices such as mobile phones, tablets and Outlook 2013 and above. With a history of over 9 years of successful synchronization with multiple backends Z-Push is the leading open source push synchronization.

• Z-Push @ GitHub.

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.

Lightweight CalDAV+CardDAV server

This is a very experimental, not complete, not fully functionnal, probably heavily bugged project. In fact, for the moment, this is closer to a proof of concept than an actual “usable” extension. But it works as long as you don’t stress it too much :).