Syncthing

Syncthing is an open source continuous file synchronisation program that keeps files identical across two or more computers in real time. It has no central server and no cloud account: devices find and sync with each other directly, and the data never passes through a third party.

That architecture is the entire point rather than an implementation detail. Every mainstream sync service works by storing a copy on a provider's infrastructure, which means trusting that provider with the data and depending on their continued operation and pricing. Syncthing removes both dependencies, because devices exchange files between themselves over encrypted connections and the provider does not exist.

The practical consequence is that there is no storage quota and no subscription, since the capacity available is whatever the devices themselves have. For anyone syncing large volumes, that is a materially different cost structure from per gigabyte cloud pricing.

The trade is that both devices must be reachable at the same time for a change to propagate, which suits machines that are frequently on and is less convenient for occasional devices. The project is run by a foundation, is free, and publishes its security model openly, which is the appropriate posture for software whose main claim is that nobody else can read your files.