Known as CK2AutoBorder in previous incarnations
This program adds one-pixel-wide black borders to pixel-art maps, such as those exported from Crusader Kings II, Europa Universalis IV, and Victoria II. The borders are continuous—i. e., the pixels composing them never are merely diagonally adjacent to each other. (You can accomplish the same effect with GIMP's Roberts or Differential edge-detection filter, if you're willing to go through an extra step or two.)
This program accepts both BMP and PNG files (in Chromium-based browsers; other browsers have not been tested). To download the output in PNG format, right-click on it and select the Save Image As option.
Due to a problem outside of my control (
(The algorithm is extremely simple: For every pixel in the input image, if that pixel differs in color from its east, southeast, or south neighbor on the input image, then that pixel is colored black in the output image. Due to the problem described in the previous paragraph, however, this implementation merely prescribes that the sum of the differences in the red, green, and blue components between any two non-border pixels cannot exceed 3/(3 × 255) without the program's adding a border.)