Wow, very nice. I would like to do this aswell!
Its terragen search it on google for the download
Just google Terragen, it should be the first link. It's free to download, but in order to use some of the more advanced features and make larger landscapes (like a farther out horizon) you need to buy a $100 license. But I made these with the free version, so you can see it is still amazing free. It's easy to use, but Terragen 2 is harder to use. I suggest Terragen 0.9, it's what I use. It's user friendly you could say.
I have done work on other programs like this before. I just got it, and have already posted up some of my work. As using this kinda graphics before, I'm finding it really easy!