Syncthing is a cross-platform peer-to-peer file synchronization client/server application written in Go.
Similar to BitTorrent Sync, the tool can be used to synchronize files between computers however, unlike BitTorrent Sync, Syncthing is open source. And it’s not just Syncthing that’s open source. The P2P protocol used by Syncthing, called “Block Exchange Protocol”, is open too. Unlike cloud sync tools such as Dropbox, Syncthing transfers the files directly between nodes (other computers on which you’ve installed Syncthing) which has several advantages: the files don’t end up on some cloud server, the transfer speed doesn’t depend on third-party servers and also, there are no space restrictions (except, obviously, the HDD space available for those nodes).