If you're only using Ubuntu or other free operating systems, then I would recommend installing http://apt.ubuntu.com/p/openssh-server. Just install it on both machines. You don't have to configure the service in any way. Then, on the client in Nautilus, select File > Connect to service.. In that dialog, choose the SSH protocol, provide username and password. You can choose to remember the password for one connection, for the desktop session or forever. If you're on a local network, then you can just use hostname.local as address (for instance, mypc.local). If you're on the internet (because SSH supports that in a secure way) you can enter the IP address.
Now you can use folders on the other PC as if they were local. You can also remote control it using that server, so it's useful in other cases too.