Wow. Sometimes you just stumble across the coolest things. Like this program, terragen. It generates realistic terrain images. It has a zillion knobs to turn. It has a pretty usable interface, considering its complexity. And the output is pretty amazing.
I've just been playing with it for a day now, and I'm really impressed. I'll probably have to start up a gallery page. For now, here's an image I cranked out:
I'm going through this tutorial in order to learn some of the tips and techniques. Man, this is one amazing program. Did I mention it's free?