While searching if there was a new version of Terragen, a 3D landscaping tool I used in the past and wanted to re-install, I've found 3D Terrain Generator. It is a small tool that improves Terragen's idea of creating in a simple way a terrain map (from a bmp for heights and textures for terrain types) by letting you control every polygon and thus allowing to modify every bit of the generated map.
And it's made with .NET!
Here are a few screenshots of the tool:
