Radicale is a small but powerful CalDAV (calendars, todo-lists) and CardDAV
(contacts) server, that:

    Shares calendars through CalDAV, WebDAV and HTTP.
    Shares contacts through CardDAV, WebDAV and HTTP.
    Supports events, todos, journal entries and business cards.
    Works out-of-the-box, no installation nor configuration required.
    Can warn users on concurrent editing.
    Can limit access by authentication.
    Can secure connections.
    Works with many CalDAV and CardDAV clients.
    Is GPLv3-licensed free software.

See also: https://github.com/Kozea/Radicale