AC3D is a 3D design program which has been available since 1994. The software is used by designers for modeling 3D graphics for games and simulations - most notably it is used by the scenery creators at Laminar Research on the X-Plane (simulator). The .ac format has also been used in FlightGear for scenery objects and aircraft models
Read more about AC3D: History, Modeling, 3D Files, Scripting and Plugins