3DSMax is the professional version of the free Gmax, and currently owned and published by Adobe. Way too expensive for most FS developers, but used by MS in the original design of FSX (and probably also by the Dovetail and Lockheed developers, with deep pockets).
Many of us have somehow got hold of (older) versions and are using these to work on FS add-on development. Most freeware developers still use Gmax however.
This page will list info on 3DSMax.
Here is the HOME PAGE of 3DSMax from Adobe: http://www.autodesk.com/products/3ds-max/overview