Allows multiple players behind a NAT (router) to connect to external game servers for games that use the old Microsoft DirectX netcode (e.g., Age of Empires, MechWarrior 4, Dungeon Seige, etc.) .  A friend had the idea back in ~2002.  I wrote a quick / dirty app.  It worked!  The code is ugly but it seems to have helped some people.  We've had 60,000+ downloads and surprisingly, 12+ years later, we're still seeing about 1500 downloads per year.  The original site and binary distribution is here http://www.puffinsoft.com/.