Calendar/Contacts Sync Tools
Discover free, open source, and secure ways to sync your contacts and calendars across your devices.
NextCloud is a suite of client-server software for creating and using file hosting services. This includes calendar sync via CalDAV and contacts sync via CardDAV. Nextcloud is free and open-source, thereby allowing anyone to install and operate it without charge on a private server.
Client OS: Windows, macOS, Linux, BSD, Unix, iOS, Android, Fire OS. Server: Linux.
EteSync is a secure, end-to-end encrypted and journaled personal information (e.g. contacts and calendar) cloud synchronization and backup for Android and any OS that supports CalDAV/CardDAV. It costs $14 per year to use, or you can host the server yourself for free.
Client OS: Android, Web. Server OS: Linux.