The primary emphasis of these packages is on artistic, synthetic terrain, although many of them have at least some limited ability to import real-world data layers.
Artifex
Terra 3D
A
history stretching back in the 1990s, very popular for many years with a
large user base of hobbyists.
L3DT
by Aaron Torpy

Terragen
/ Terragen 2
Voxelogic
Acropora
WorldCreator


World
Machine
Artificial
Planet
FreeWorld3D
(freeworld3d.org)
Was $480
/ $200 academic, from Pandromeda, Inc., for Mac or Windows
TechNature
(http://www.tanzanite.to/)
WorldForge