Baïkal is a lightweight open-source CalDAV and CardDAV server for synchronizing calendars and contacts. It is designed for simple self-hosted deployments.

EteSync provides end-to-end encrypted synchronization for calendars, contacts, and related personal data. It is designed to keep synchronized information private from the service provider.

Fossify is a collection of open-source Android applications focused on privacy and simple, functional alternatives to common mobile apps.

Morgen is a calendar and productivity platform that combines calendars, scheduling, tasks, and related workflow tools in one application.

Proton Calendar is an encrypted calendar service from Proton. It is designed to protect calendar data while providing scheduling and synchronization features.

Tuta Calendar is an encrypted calendar service from Tuta. It focuses on privacy and secure synchronization alongside Tuta’s other services.