Syncthing is a decentralized open source tool to synchronize files across multiple devices without using third party cloud servers like Dropbox, what should be an objective for people who care about privacy.
Data in Syncthing is transmitted peer to peer via TLS encryption with perfect forward secrecy directly to your other devices, it never touches the Internet where it could be intercepted, only nodes you have previously authenticated are able to connect.
Other advantages or running your own cluster are that there is no storage space restriction other than your own drive, you will not be reliant on a cloud service that could not be available when you need it and data transfer is speedy.
Dropbox alternative Syncthing
Syncthing is cross platform, it works in Windows, Mac, Linux, BSD, Solaris, Android, iPhone, it can be installed on any computer, server or mobile device you own. This tool is similar to Bittorrent Sync with the difference that everything is open source, including the protocol used to synchronize files, called Block Exchange Protocol.
Machines are identified with an ID, when you add a node ID to the network, any folder listed in the repository starts to synchronise downstream, files are split into blocks for easier transmission, the more devices are connected, the quicker everything will sync as more download sources are available.
There is one downside to running your own cloud, if you wish to publicly share files over the Internet, it can be done but you have to be tech knowledgeable, you will have to combine it with something like Cloudup or Freehold, it is not supported by the developers. If you often share files over the Internet, it is best to download to Owncloud, which needs a server, whereas Syncthing can be run on any desktop computer.
I liked the open source nature of this tool as well as the support it has for all operating systems, it is more complex to use than BT Sync but it gives you more control over how you share files on a network.