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

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.

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