I started playing with World Machine Basic today. It’s a free tool for generating believable terrain.
It uses procedural algorithms to create or modify a height map and then simulates realistic erosion on it to achieve a very natural looking terrain surface. It’s the most fun I’ve had with a new tool in a long while. In fact, I was so entertained and impressed that I sprung for the reasonably priced full version.
Next step: exporting height maps and masks to use in a game engine