Terragen - Terragen is a scenery generator, created with the goal of generating photorealistic landscape...
Vue - Create vast expanses of terrains, add trees, select the best point of view and render...
Picogen - The homepage of the free (as in freedom), GPL-licensed Terrain Synthesizer and Renderer named 'picogen'
DreamScape - Plugin for 3ds Max that creates skies, clouds, ocean waves, boat wakes, naval dynamics and terrain.
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.