I found myself wanting this a lot when doing early dev on any kind of tile-based game. These are mockup/placeholder tilesets that allow you to draw floor/walls/platforms and various shapes to represent game objects or players. The shapes have an outlined form and a pure white form which makes them easy to recolor as texture regions. There is a 32x32 version and a 64x64 version, as well as a demo of how to set them up in Tiled.