A couple of weeks ago I came across the Worldwind mapping program from NASA. Not sure whether it was in of the forums here or not. At first glance it was just another Google Earth – until I discovered the add-ons! Underneath a simple screenshot from the program with ‘Demis Worldmap’ add on.
Worldwind Mapper - Screnshot
You will note that this shot includes clearly visible rivers which are kind of difficult with Google Earth (or I haven’t figured out how). I have always found that placing rivers without a proper template is a lot of guesswork (which those perfectionists really hate) and takes a lot of time.
With this kind of screeny all you need to do is size it into the map_heights format and the map_features format of your map and then layers is your best friend.
Here are the links:
[Tutorial] Creating a World – Working with the Worldwind mapper
Online Mapper (updated 08.08.2010 - thanks IZob)
The new online mapper includes the "Biome" settings which can be used for map_climates. Unfortunately the individual biome are seperated by a black line, so some small work is required coloring that into the correct RGB value.