Syncthing replaces proprietary sync and cloud services with something open, trustworthy and decentralized. Your data is your data alone and you deserve to choose where it is stored, if it is shared with some third party and how it’s transmitted over the Internet.
Interesting. Unlike the open source ownCloud that does rely on a central server this appears to use a peer to peer strategy to sync data between devices you select with no hub server. See Syncthing’s Getting Started page for more details. The whole thing is open source and available on Github.