Best software for drawing diagrams?

[rprosperi]

If ASCII art is sufficiently detailed to portray your images, then consider using this website, which provides a dedicated draw program for creating ASCII art, with objects (squares, arrows, etc.), an eraser, text boxes, etc. It's much easier to use than explain as I've not seen a tool like this before.

http://asciiflow.com/

When you later upload you 'art' you should sure to either wrap it in code blocks, or manually select a fixed-pitch font to preserve the uniform character width so the images retain their fixed columns.

Not as good a result as a dedicated drawing program for sure, but it is free, trivial to use and easier to paste into posts than inserting image files.