Syncthing
Syncthing's homepage pitches it as an open-source, peer-to-peer tool that keeps folders in sync across your own devices or with other people, with no central server holding your data. It rests its case on three claims: privacy (files live only on your machines), TLS encryption with perfect forward secrecy, and certificate-based device IDs so only devices you approve can connect. It also stresses ease of use, with a browser-based interface, support for most major operating systems, and setup that avoids manual IP or port configuration by relying on device IDs and UPnP. It notes that Kastelo provides commercial support and development sponsorship.