A VPN client, done right
Freelan is a free, open-source, multi-platform, peer-to-peer VPN software that abstracts a LAN over the Internet. It works on Windows, Linux and Mac OSX.
Whether you want to connect the computers of your family, play an old LAN-only game with your friends, or give a privileged access to your private network to your collaborators, freelan will do the job perfectly.