In the early days of the Mac, I used to love playing around with Bryce, which could generate very realistic scenery, which was my forte, or bizarre other worldly scenes straight out of science fiction flick.
Bryce is still around, along with high-end rendering programs such as Vue. These programs aren't cheap. Bryce is $249.95 for the Pro version and Vue has versions from $99 to $995 but they have amazing features and power.
I've been playing with TerraRay 4, which currently is on sale for $4.99. It's a great way to explore landscape creation on a budget. This app doesn't create animations like its high-end cousins, but it has some easy-to-use tools that will let you create some stunning landscapes with not too much effort.
The app allows you to control lighting, the materials that make up the surface of your creations and can produce realistic skies and fog. It also allows you to import models in the popular 3ds file format. You can learn more at the developer website and see a video of the app in action. The app contains complete documentation.
I didn't do anything too fancy, but was able to create some canyons with water and decent looking skies with ease. The only real downside is the final rendering speed. For a large scene it might take an hour to do all the detail and ray tracing. That's just far too long for the size of file that is being output. One fairly simple render in TerraRay 4 took more than 30 minutes. Of course, it can be done in the background, and the final result looked fine, but this program really needs to be optimized for speed. The program only saves in .png format, which seems a bit shortsighted, and other file formats would be useful.
Check out the app at the developer's website and see if it suits your needs. I also have some images in the gallery. I would say TerraRay 4 is powerful, inexpensive, but dog slow. Don't expect it to be as good as the big boys, but it more than holds its own. I recommend this app if you are getting your feet wet with synthetic landscape generation. The app needs OS X 10.6 or greater, it functions fully under Lion, and is a 24 MB download.