In no particular order:

World Machine
Wilbur
Leveller (Daylon Graphics)
L3DT
GeoControl
Fractal Terrains
Instant Islands
Bryce
Terragen
Mojoworld
Vue
VTP
Grome
...I'm sure there's more...
in fact you can also include 2D image editors such as Photoshop, GIMP, etc.

free progs:
Wilbur
Instant Islands
GIMP
VTP

I'd recommend Wilbur: free and does loads. 3D view is not that great in my experience but it's always been one of my go-to apps.

Hope this helps!

monks