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!
ShapeShop - ShapeShop is an advanced and professional 3D design and modeling application that comes with an easy-to-use and user-friendly interface.
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.
Misfit Model 3D - Misfit Model 3D is a 3D model editor that basically operates with triangle-based models.