Or Syncthing, which does very much the same thing as BTSync -- multi-way sync of one or more directories between several computers you own/control. Syncthing is

Or Syncthing, which does very much the same thing as BTSync -- multi-way sync of one or more directories between several computers you own/control. Syncthing is free software, and offers better security guarantees than BTSync, but as a result it's slightly harder to set up a folder on each client. Or there's also OwnCloud, which gives you a very simple user experience; you just need to have a centralized storage server set up.