InkOnScreen is an unusual tool which allows you to draw directly on your desktop, perhaps useful if you want to add some kind of annotation before you take a screen grab.
Once launched the program adds an icon to your system tray, and runs in the background. Its 20MB memory consumption is a little more than we would expect, though not enough to make any real difference to most people.
When it's time to draw, simply click the icon and your mouse cursor turns into a "pen": just click and drag to draw anywhere on your screen.
A small tool palette provides more options. You can set the brush size, shape and colour, for instance. There's an option to draw freehand shapes, then resize and reposition them. You get an eraser, an Undo tool, a "copy to clipboard" option, Save to JPEG and more.
The implementation of this is extremely basic, for the most part. It's also not entirely clear what some of the options do, as there's no documentation at all. And arguably you'd be better off capturing your screen as usual and annotating it in a proper editor.
If it might be useful to draw on your desktop in real time, though - perhaps if you're training people, or giving a presentation - then InkOnScreen may be able to help, and it's certainly worth a try.