I've often used a tool such as Ultimate Pen to draw all over a database screen during a presentation. I've been frustrated, though, at not having an easy way to "wipe the slate clean" and get a blank background, like a "whiteboard" for a quick diagram, or text "playspace". The fact that I can choose background color, and can get this to blank the screen immediately upon launch, make it convenient to invoke quickly (say, with a hotkey by way of HotApp or LaunchBar, etc.). These two features make it more convenient than the otherwise similar "Blank This Screen" (though it seems that one is aimed at multiple-screen situations, and I don't have evidence whether Screen Blanker can blank just one display or another selectively.