Syncthing

Syncthing is a free, open-source, peer-to-peer file synchronization utility: it connects your devices to continuously and securely sync data without a central cloud server.

Syncthing delivers robust, continuous file synchronization across multiple devices (Windows, macOS, Linux, Android, etc.) with a zero-knowledge, peer-to-peer architecture. Your data remains private: it's never stored on a third-party server. All communication is secured using TLS encryption and perfect forward secrecy. Devices authenticate via strong cryptographic certificates, ensuring only explicitly approved nodes—identified by a unique Device ID—can connect. This open-source system is powerful enough to manage numerous shared folders between your personal machines or a multi-user team, operating seamlessly over both LAN and the public Internet.

https://syncthing.net/