Terragen - Terragen is a scenery generator, created with the goal of generating photorealistic landscape...
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!
Picogen - The homepage of the free (as in freedom), GPL-licensed Terrain Synthesizer and Renderer named 'picogen'
Vue - Create vast expanses of terrains, add trees, select the best point of view and render...
Hexagon - Hexagon - Box Version
World Creator - World Creator is a real-time terrain and landscape generator and creation tool, which is industry proven by many game, movie and artwork companies.