Syncthing

Syncthing replaces Dropbox for people who only wanted the sync, not the storage. Point two or more devices at the same folder and they keep each other up to date, directly, with nobody in the middle.

There is no account, no monthly fee and no size limit beyond your own disks. Transfers are encrypted, and it works across the internet as well as at home, so a laptop away from the house still keeps up.

Two honest warnings. The controls live in a web page that opens in your browser, which feels odd at first, and the version on Flathub, called SyncThingy, is a community-made package that bundles Syncthing with a small tray icon rather than something published by the Syncthing project itself. Also remember that sync is not backup. If you delete a file by mistake it disappears everywhere, so keep Déjà Dup or Pika Backup running as well.