Syncthing

syncthing.net

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Open-source, peer-to-peer file synchronization — sync folders directly between your own devices with no cloud server or subscription.

Why it is useful

Syncthing transfers files directly between devices over an encrypted connection, so there's no third-party server storing your data and no storage-tier subscription. It's cross-platform (desktop, NAS, mobile) and handles conflict resolution and versioning automatically, covering most of what Dropbox is used for.