For those who havent heard of it, this is a small program which allows you to create a Local Area Network over the internet. Its pretty nifty to use to play LAN based multi-player games on the internet against friends only.
I use it to play a game called Machines over the internet with my friends since it doesn't support TCP/IP and theres no server for it.
Very simple and easy to use, i can connect to my friends computer and then connect to the Darwin Wireless network through his computer :D, you can also see everyones shared folders using this program, but you can disable it too :p
http://www.hamachi.cc/