Cool tool of the day: pwnat. From the description:
pwnat, pronounced “poe-nat”, is a tool that allows any number of clients behind NATs to communicate with a server behind a separate NAT with *no* port forwarding and *no* DMZ setup on any routers in order to directly communicate with each other. The server does not need to know anything about the clients trying to connect.
Simply put, this is a proxy server that works behind a NAT, even when the client is behind a NAT, without any 3rd party.
There is no middle man, no proxy, no 3rd party, no UPnP/STUN/ICE required, no spoofing, and no DNS tricks.
The above page also features a description of how it works. Very cool.