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Radicale
A lightweight self-hosted calendar and contacts server, the CalDAV/CardDAV equivalent of iCloud or Google Calendar.
Radicale syncs calendars and contacts across phones and computers the exact same way iCloud or Google Calendar would; the apps don't even know the difference. The difference is where that data actually lives: on this server, not in Apple's or Google's.
It's deliberately simple software: no bloated web interface, no upsells, just the sync protocol working quietly in the background.