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.

fruux is a cloud synchronization service for contacts, calendars, and tasks based on open standards such as CalDAV and CardDAV. It supports synchronization across multiple devices.

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.