(the link again in case the clickable title doesn’t work on all instances: https://sketchtoy.com)

Have you ever tried to explain something over the internet that doesn’t translate well into words (like a specific route on a map, the layout of a building etc.) and you also can’t find a good picture on the internet showing what exactly you mean? Then just use that site to doodle a sketch. It is free of charge, doesn’t require a registration/verification/whatever or extra software, and the sketches seem to stay online forever.

I have an online roleplaying group that relies solely on chat / texts and this site has helped me numerous times to demonstrate the layout of a tavern, camp or dungeon, to show what in-game items look like and/or how big they are, and to add easy-to-understand visuals to in-game puzzles that would otherwise be hard to describe verbally.