Terragen - Terragen is a scenery generator, created with the goal of generating photorealistic landscape...
Unity - The multiplatform game creation tools for everyone.
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!
Godot Engine - Feature-packed 2D and 3D open source game engine.
Picogen - The homepage of the free (as in freedom), GPL-licensed Terrain Synthesizer and Renderer named 'picogen'
Blender - Blender is the open source, cross platform suite of tools for 3D creation.