Chanjmap Map Editor
This is a simple map "editor" from our friend Stewart over at the xcomufo.com forums. It allows you to change individual 10x10 or 20x20 map blocks or an entire terrain to a number of preselected types. It will also make a backup of your MAPS and ROUTES folders but don't worry about the warnings - it works fine.
This program has been modified (with permission) from the original by Zombie so that you no longer need to change the file extention type and removes the installer. Simply extract into your main Xcom game directory and double click on "chanjmap.bat" to run.
For more information or to discuss this program with the author, visit this topic over at the xcomufo.com forums.