Radicale

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.