Syncthing
Open-source, continuous, peer-to-peer file synchronization — no central server required.
Syncthing keeps files in sync across your own devices continuously, without routing them through a central server. That’s the key difference from something like Nextcloud or Dropbox: it’s peer-to-peer and decentralized by design — your devices talk directly to each other over an encrypted connection, and there’s no third party’s server holding a copy in the middle.
It’s open-source and privacy-focused from the ground up, which makes it a natural fit for keeping something like device backups or shared folders in sync between a laptop, a phone, and a homeserver without introducing a cloud provider into the chain at all.
Worth knowing: this site’s own hosting infrastructure runs Syncthing as a real, working service.