Terrapin
Some time ago, inspired by the landscape generator 'Terragen', I wrote a program which I named 'Terrapin'. It generates fractal terrains, and raytraces them. There is a real-time preview window in which you can roam around the landscape, adjusting parameters via a rather complex set of dialogs.
When you've got it set up as you like it, you can render a larger scale image, and save it out as a bitmap. Typically in about 10 seconds.
Features include:
Terrain height and colouration
Atmospheric effects (fog, haze, clouds, sky colouration)
Stars
Water with waves and true reflections
Lens flare
Etc. etc.
Oh, and it also has a mode that allows you to create whole planets viewed from space.
Sample landscapes images
Sample planet images
Home
This is what it looks like when it's running: