For example 3D Studio and MAM-Virtual Reality System belong in /Computers/Software/Graphics/3D while Animatek World Builder, World Construction Set, Terragen, and Virtual Terrain Project can all live here in 3D/Terrain.
There is a separate category for GIS based terrain modellers at http://dmoz.org/Science/Social_Sciences/Geography/Geographic_Information_Systems/Software/Visualization_Software/