I've been using this awsome little program for a few weeks now and its still impressing me. It's a VPN program that lets you play games online with friends with the game thinking your all on a LAN. No need to forward ports or worry about NAT hamachi will work regardless.
I came across it after a friend and I spent 45 mins trying to get his router to forward the correct ports to play, only to have it work for 5 mins then fail (thanks netgear). A tiny download, and 30 seconds of configuration and we were playing without problems for hours, and game that has LAN network gaming will work like a dream. I highly reccomend it for anybody throwing together a last minute game of anything.
Heres the link, enjoy http://hamachi.cc/download/
Off to play some co-op HL2