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.
primarily
meant for creating abstract, arty landscapes, still very popular with a very
large user base of hobbyists
FreeWorld3D
L3DT
by Aaron Torpy

$480
/ $200 academic, from Pandromeda, Inc., for Mac or Windows
Terragen
/ Terragen 2
Voxelogic Acropora


World
Machine
Artificial Planet
includes a GUI editing environment and a DLL which can be used by other
applications
TechNature
(http://www.tanzanite.to/)
WorldForge