Syncthing

Syncthing is for the specific case where you pay Dropbox to keep one folder identical on a laptop, a desktop and a machine at home, and you never once share a link with anyone. It connects your devices to each other directly, encrypts what passes between them, and keeps no copy anywhere else. No account, no quota, no monthly line on the card.

Setup is not an app install. You run it in the background and configure it in a browser at localhost, pairing machines by long device IDs and then setting rules per folder: send-only, receive-only, ignore patterns, how many old versions to retain. Once paired, it is genuinely unattended for years.

It is not a backup, and that is where people get hurt. Delete a file on one machine and it vanishes from the others, exactly as designed — turn on file versioning before you trust it with anything irreplaceable. Devices also have to be awake simultaneously, so a laptop that slept for a week catches up when it wakes and not before.

If what you actually want is a drive in the cloud you can open from a phone browser in a hotel, keep paying Dropbox. If you want your own computers to agree with each other, this does the whole job. Kastelo sells paid support for it, if free software with no support desk is a dealbreaker.