Can be a bit finicky to setup, but IMO no better way to sync between devices - especially when you're offsite w/ 2 devices on the same network, & don't have the bandwidth to upload & download an entire copy to a centralized server, as would be required with dropbox & other similar options.
Syncthing is a great solution to keep files between devices in sync. For me it has replaced Dropbox and other similar services. It works like a charm, free, and open source.
The only thing that I am missing is iOS support, but that's because Apple doesn't allow for creating effective iCloud competitors on the platform, not the Syncthing authors' fault.