I just found this amazing application that automagically synchronizes folders on 2 different devices over a LAN or even internet. You just have to set the devices up one time and in syncs files put into those folders. It’s called Syncthing.
Previously I had used a similar app called BTSync, but those people were cagey and not very upfront about their code. This app is all open source and based on well documented technology. Check it out: