Terragen - Terragen is a scenery generator, created with the goal of generating photorealistic landscape...
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!
Sculptris - Sculptris: Enter a world of digital art without barriers.
MagicaVoxel - A free lightweight GPU-based voxel art editor and interactive path tracing renderer.
meshmixer - meshmixer is an experimental 3D modeling tool for making 3D mashups without too much hassle.