Anyone like me who has a calibrated CRT monitor knows the problem: when you're not doing image editing, the monitor is set way too bright, especially as it gets darker. But you can't adjust the contrast control or especially the brightness setting — the calibration only works correctly if the luminance is set properly!
This little utility comes to the rescue: just start it and you get a small graphical slider with which you can adjust the screen brightness. Works splendidly. And as soon as you're working with images, you can stop it beforehand (and give your eyes some time to adjust, so your brain can adapt and everything doesn't look wrong).