World Machine - Command powerful tools like erosion and advanced colormaps to create terrain heightmaps, meshes, and textures for your game or 3D scene. Download for Free!
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
Vue - Create vast expanses of terrains, add trees, select the best point of view and render...
ZBrush - ZBrush is a digital sculpting and painting software solution.
Picogen - The homepage of the free (as in freedom), GPL-licensed Terrain Synthesizer and Renderer named 'picogen'
Cinema 4D - Cinema 4D is a 3D modeling, animation, motion graphics and rendering application.