Radicale

Radicale is a calendar and contacts server you run yourself. When you want your events and address book off Google or Apple but still synced across your phone, laptop, and desktop, this is the piece that does it. It speaks CalDAV and CardDAV, the standard protocols every major calendar and contacts client already supports, and it stores everything as plain files on disk. No database, no heavy stack.

It is famously easy to stand up: a Python install and a small config, and you have a working server. Production hardening is where it climbs to moderate, since you will want TLS, real authentication, and a reverse proxy in front of it. But there is no database to babysit and the data is just files you can back up with anything.

Radicale is GPLv3 and completely free, with no paid tier or hosted option. The trade is that you become the host. Baikal is a similar single-purpose option, and when you want calendar and contacts as part of a bigger self-hosted suite with a web UI, Nextcloud or SOGo cover more ground. Solo users and families are the sweet spot. Small teams can run it happily; larger orgs usually want the broader suite instead.

The catch is scope. Radicale does calendar and contacts and nothing else, and there is no friendly web app for end users to click around in. You manage everything through a client, and uptime and backups land entirely on you.