Syncthing

Syncthing syncs folders between your own devices — peer-to-peer, encrypted, with no cloud middleman. Set it up once and files flow automatically between your laptop, desktop, and NAS. It handles conflicts gracefully and works over LAN or the internet. The official GUI wrapper (Syncthing macOS Application or SyncTrayzor on Windows) makes it feel like Dropbox without the Dropbox. Free, open-source, and infinitely more private than any cloud sync service.