Blender - Blender is the open source, cross platform suite of tools for 3D creation.
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!
ZBrush - ZBrush is a digital sculpting and painting software solution.
Terragen - Terragen is a scenery generator, created with the goal of generating photorealistic landscape...
Cinema 4D - Cinema 4D is a 3D modeling, animation, motion graphics and rendering application.
meshmixer - meshmixer is an experimental 3D modeling tool for making 3D mashups without too much hassle.