LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
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!
FreeCAD - An open-source parametric 3D modeler
Vue - Create vast expanses of terrains, add trees, select the best point of view and render...
SketchUp - 3D for Everyone
Picogen - The homepage of the free (as in freedom), GPL-licensed Terrain Synthesizer and Renderer named 'picogen'