xannotate is a simple and open source program that will open a screen-size window over whatever is present and will allow you to scribble over it. This is useful when you are doing a demo and you need to annotate parts of what is on-screen.
xannotate takes no arguments and start immediately, without feedback. Use the mouse`s Button 1 to scribble. Press the Escape key to exit silently.
Most typically, you would bind a function key in your window manager to invoke this during a demo.