Draw.iO Diagrams is a cross-platform diagramming application you can use to create flowcharts, workflows, org charts, Venn diagrams, wireframes, network diagrams, and just about any other chart or ...

it’s great for a quick-and-dirty circuit diagrams. Not exactly schematics, but [Duckman] has some Arduino pinout diagrams he made using ASCII-art. These could be useful when pasted into source ...