Hamachi is a VERY interesting piece of software. It is a mediated (it has some public relay servers for getting initial sessions established) peer to peer VPN software client. It allows you to dynamically build out VPN sessions behind almost all NAT devices to other machines as if they are on the local lan with ZERO configuration on your part. As long as you know the name of the network you want to join and the password - like magic - you are connected. It is using UDP and works on Windows 2000/XP and Linux with an OS/X version coming soon.
I have to admit that I am impressed with this project. Easy to use VPN for peer to peer projects. Very cool.