ZDoom Terrain Generator + help
(includes demo wad & help)
What is it? It's a tool that makes terrain from a grayscale heightmap. It generates map full of triangle sectors & slope things which makes the terrain. You can then open the wad with your favorite editor and copy & paste the terrain into your level. You can actually test the generated map itself, but you have to build nodes and optionally set some textures.

Known bugs: It's not stupid-proof. E.g. if you make the heightmap too large (so the level width or height will be > 65536), the map will be screwed up.