This program makes images of landscapes digitally. Good for rendering skies especially. "Terragen™ is a work-in-progress scenery generator for Windows and the Mac OS. At this stage of its development, Terragen is free for personal non-commercial use." Terragen™ is a powerful solution for rendering and animating realistic natural environments. Create entire worlds from your imagination, or import real world terrain datasets and use Terragen 3 to create the most realistic visualisations possible.
Terragen 3 (Free/Paid 3D World Rendering Program)