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

Shares calendars and contact lists through CalDAV, CardDAV and HTTP. Supports events, todos, journal entries and business cards. Works out-of-the-box, no complicated setup or configuration required. Can limit access by authentication. Can secure connections with TLS. Works with many CalDAV and CardDAV clients Stores all data on the file system in a simple folder structure. Can be extended with plugins. Is GPLv3-licensed free software.