Still in use, the ‘old’ design program once made by Discreet and the base tool for the MS Flight Simulator developers. The last version made by them is 1.2, and it still is available (for free now) AND widely used by add-on FS developers.

The official MS development tool from FSX onwards was of course Autodesk’s 3DSMax, in all its variations and versions. Too complex (maybe) but above all too expensive for all but the wealthiest add-on developers.

You would typically need a ‘pro’ version of FSX, which includes the MSFS SDK, which in turn includes Gmax.

The Basics

GMax 1.2 is available here on Turbosquid.

Installing and using it for FSX is not so simple.
FSDeveloper FSX SDK Installaton Guide here.

Other Links

Milton Shupe, one of the world’s foremost freeware aircraft designers, has published a wonderful tutorial on how to use Gmax to build an FS2004 aircraft!
Click here for Milton’s Gmax Tutorial.