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:
Damm, I need free time to try all these things!