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 ...

The project was inspired by forum member Graynomad's Arduino Due diagram, so be sure to grab that one as well.

QElectroTech is an open source tool for drawing these types of diagrams. It consists of two tools: an element editor for creating schematic symbols and a diagram editor for creating your drawings.