Terragen is a landscape generator for Windows and Mac (Classic and OS X). Very interesting piece with a bunch of parameters that you can adjust. And as it seems quite fast. I have to play with it a bit.

Yes, it really works very simply. Ok, generating a sample landscape is of course rather randomly controlled - to generate exactly what you have in mind requires much more work. But the whole thing actually gives the impression that you could understand it. The colors come across a bit cool, so I did a slight white balance adjustment towards warmer colors in Photoshop. It also looks a bit harsh and the sharpness between water and land is a bit too blurry for me, but I guess one or the other can certainly still be adjusted with parameters.

What I noticed with the first experiments: you should stick closely to the physical model. So if the camera position is set to 30 meters above the ground, it also looks that way - you should first think about where the viewer is in your scene. In real situations, you rarely hang 30 meters in the air. You should also play a bit with the 3D preview, as it offers a simple way to optimize the viewing angle.
Unfortunately, there are hardly any plugins for OS X - a plugin that simulates a camera lens both in terms of the angle of view and depth of field would be great. But there is only one for Windows.
What is also not so nice - the vegetation is only simulated by colored spots. Just like snow and sand. Some of the people in the gallery images seem to use their own textures or better presets than those provided. So far I have only used the pre-made surface textures. I need to research further how to get better presets that look more realistic - the ones provided always look a bit thin.
Of course, the presets for extraterrestrial scenes are cool - e.g. the atmosphere presets and the surface textures for Mars. You just have to paint a robot in and you have your own NASA mission.