Unison file synchronizer works like a mashup of Dropbox, rsync and Git, although internally working via its own means. Unison attempts to remotely sync changes in files between a local and (typically) remote location. We have used Unison to manage systems on isolated remote networks. That is, where the remote network is isolated from directly accessing the Internet and so Dropbox etc. aren’t available.
Assuming one trusts Unison, this could be useful for cases where it is desirable to keep files out of the “cloud”. Unison file synchronizer has executable downloads for popular operating systems.