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.
Vue - Create vast expanses of terrains, add trees, select the best point of view and render...
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
Picogen - The homepage of the free (as in freedom), GPL-licensed Terrain Synthesizer and Renderer named 'picogen'