I used Dia, a diagramming tool, to create the map below.

It's very easy to use it, to create a map like this one, I used the line and arc tools and I set the "snap to grid" option on the preferences menu. To create a grid, I created a separate layer with the lines in it. The Dia source file for this image is here.

I have submitted a feature request to their developers asking for the option to print/display the grid be added to preferences menu. If anyone wants to add a comment to that request it would be appreciated.