There are a bunch of tools that are built along with the main executable that are useful for testing various parts of the game, or building graphics/sound effects:
atlasassembles a texture atlas from a number of PNG images, while preserving their palettes.
deanimatortakes an animated GIF and turns it into a spritesheet, again, preserving palettes.
base-buildertakes a "base" tileset and builds all 256 variations of it.
tilatorconcatenates a number of tileset images into a single image
base-tile-testtests "base" tilesets by letting you draw with them on a simple map.