I think most of the members use either Gimp or Photoshop. Maybe Illustrator to some extent. I don't think there is a "mapping software" that makes it a little easier. If anyone could correct me on this please do.
It might be a stupid suggestion but have you considered using pen and paper? Sketching is great for making rough maps and I know that many of the more talented members combine sketching and digital art to make their maps.