Create a Heightmap

Use BlockMan2D .NET Framework 4.0 Required
BlockMan2D download link

Or you can use the Terrain Generator to create and modify a heightmap and colormap with more detail and save it to slab6 .vox
Terrain Generator download link

Or open or create your map in GIMP 2.6 to save it in 512x512 bmp with 24 bit color then create your map (vxl) with pyspades map editor
PySpades Map Editor download link
GIMP 2.6 download link

Or create your colormap in heightmap2colormap with your heightmap (3.4 MB)

Or create a heightmap with L3DT
L3DT download link