Radicale

Radicale is a self-hosted server for calendars, to-do lists, and contact books. It speaks two open protocols: CalDAV (for calendar data) and CardDAV (for contacts). Once Radicale is running on your own machine or a rented server, any compatible app on your phone or desktop can connect to it and keep your data in sync automatically. Most mainstream calendar and contacts apps already support these protocols, so you do not need to install anything special on your devices.

The design philosophy is minimal. There is no separate database to install. All data is stored as plain files in a regular folder on disk, which makes backups straightforward and the contents easy to inspect or move. Getting started does not require reading through pages of configuration.

Radicale supports calendar events, to-do tasks, journal entries, and contact cards. It can optionally require usernames and passwords to restrict who can connect, and it can encrypt traffic with TLS if you want to secure connections from outside your local network. Both features are opt-in, so a basic personal install stays simple while a shared family or small-team setup can be locked down properly.

The project can be extended with plugins for anyone who needs custom behavior beyond what ships by default. It works with a wide range of CalDAV and CardDAV clients across different platforms.

Radicale is free and open-source software, released under the GPLv3 license. The README is intentionally short and links to a dedicated documentation website and a community wiki for setup instructions, troubleshooting, and advanced configuration details.