If you don’t have access to Visio or AutoCAD, OpenOffice Draw is a free* program that you can use to create signal flow drawings. One of the best features is being able to make a connector between objects, which lets you reposition items on the screen while maintaining connections.
Check out the video to see how I use OpenOffice draw:
*check the licensing requirements of OpenOffice draw, as your usage might require a paid version.
What is your favorite drawing tool, and is there a single feature that makes it more useful to you?