EteSync
If you decide to self-host EteSync you won't have to pay for the subscription. Tutorial here .
Self-hosted Calendars
The Calendars we recommend, filtered to Self-hosted. See all Calendars →
Radicale
A simple self-hosted CalDAV and CardDAV server for calendars and contacts, written in Python and licensed under GPLv3.
Peergos
Audited by Cure53 in May 2019.