RPG Battlemap Generator
This is a simple jQuery tool for visualising Battle Maps using the free tileset provided by MadCowChef.
It's not terribly efficient as it loads all the images at full-size. It's not canvas based or particularly fancy. I'm pretty sure it'll crash weaker desktops. Ideally it should only be used for quick-and-dirty style map planning.
If you're looking for a piece of software specifically for making Battle Maps and find that GIMP or Photoshop are too cumbersome I would recommend checking out the Tiled project.
If you are looking for a way to make Battle Maps for use in Tabletop Simulator then please check out my Battle Atlas project.