Hamachi is a rather cool application and service for creating secure peer-to-peer networks over the Internet. Every node is assigned an unique IP address on the Hamachi network (they use the 22.214.171.124 network) and easily connects point to point, even behind two different NAT routers!
It works really great, and the security is well documented. The client is available for Windows and Linux, with a Mac version on the way.
Steve Gibson and Leo Laporte had a lengthy discussion on the Security Now! podcast last week.