Syncthing

Syncthing is an open source continuous file synchronization program. It synchronizes files between two or more devices in real time, securely and privately. Unlike cloud storage services, there is no central server. Your data is never stored anywhere else, and everything is encrypted with TLS. Syncthing runs on Windows, macOS, Linux, FreeBSD, and even on NAS devices. It can also be used on Android via a third-party app.

Setup is straightforward: install the client on each device, add a device ID, and choose folders to share. The web-based admin interface shows connection status, transfer history, and lets you manage ignore patterns. Syncthing supports versioning, folder-type settings (send only, receive only), and relay discovery when direct connections fail. It uses a block exchange protocol with delta copying, so only changed parts of files are transferred.

Syncthing is maintained by the Syncthing Foundation and is completely free with no paid tiers or hidden limits. It is licensed under MPL 2.0. The project has a large community and documentation, including a user forum and extensive guides. For anyone who values privacy and wants to sync files without relying on a third party, Syncthing is a solid choice.