Blender - Blender is the open source, cross platform suite of tools for 3D creation.

Terragen - Terragen is a scenery generator, created with the goal of generating photorealistic landscape...

Sculptris - Sculptris: Enter a world of digital art without barriers.

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!

meshmixer - meshmixer is an experimental 3D modeling tool for making 3D mashups without too much hassle.

Vue - Create vast expanses of terrains, add trees, select the best point of view and render...