I'm surprised that nobody has mentioned Syncthing yet. I use it to synchronize text documents between my devices, automatically, without any external server. It works just as well when all devices are in the same network, or I'm not at home. As soon as I tidy my pdf and image collections up, I'll be adding them too, albeit without version control.

The major downsides would be

• Requires at least two nodes to be active to synchronize (this wasn't too much of a issue for me, since I have one node that's always on)
  
• It's still under development, so there are a few bugs and changes that pop up from time to time
  
• the android client is very disapointing
  
• one has to put some thought into setting the network up to avoid dead or duplicate nodes