Bittorrent Sync is a free application that will let us to sync directories over the internet. It does not store any files on the Cloud. All files reside on our own computers or devices. BitTorrent Sync will only synchronize between them. The official Bittorrent Sync developer does not provide the GUI for their application. It uses web interface by default. But, thanks to Leo Moll who provides the Bittorrent Sync GUI PPA. So we can easily install Bittorrent Sync on Ubuntu based system such as Elementary OS Luna.
How to install Bittorrent Sync GUI on Elementary OS Luna
sudo add-apt-repository ppa:tuxpoldo/btsync
sudo apt-get update
sudo apt-get install -y btsync-gui