Blender - Blender is the open source, cross platform suite of tools for 3D creation.
Terragen - Terragen is a scenery generator, created with the goal of generating photorealistic landscape...
Sculptris - Sculptris: Enter a world of digital art without barriers.
Vue - Create vast expanses of terrains, add trees, select the best point of view and render...
meshmixer - meshmixer is an experimental 3D modeling tool for making 3D mashups without too much hassle.
DreamScape - Plugin for 3ds Max that creates skies, clouds, ocean waves, boat wakes, naval dynamics and terrain.