Syncthing is a continuous file synchronization program. It synchronizes files between two or more computers in real time, safely protected from prying eyes.
Syncthing is a great solution to keep files between devices in sync. For me it has replaced Dropbox and other similar services. It works like a charm, free, and open source.
The only thing that I am missing is iOS support, but that's because Apple doesn't allow for creating effective iCloud competitors on the platform, not the Syncthing authors' fault.
