From Nem's Tools:
Nem's Mega 3D Terrain Generator is a relatively new terrain generator that takes a step in a completely different direction. Unlike traditional generators where the user must depend solely on the generators generation abilities, this generator gives its users complete 3D control over what their terrain looks like. Users start off with a flat piece of landscape, then add to it with one, or a combination of many powerful generation options. They then have several tools at their disposal to literally grab the terrain and pull it as they want it. The terrain can be saved as a .tgm file for latter use and exported to a .map file for use in any map editing programs for Quake engine based games such as Half-Life. Additionally terrain can be exported as a .t3d for use in UnrealEd or a .rmf or .vmf file for use in Hammer or any number of other supported formats.