For one reason or another I’m always measuring images and other screen real estate in web pages or elsewhere. This recently got a lot easier when I discovered the MeasureIt add-on for Firefox. (Which fits snugly next to ColorZilla, the equally useful in-browser color picker.)
The “elsewhere” part has remained difficult until recently when I stumbled across the Wonderweb Screen Ruler. I find that I will leave it open and on top of the screen for easier use when working with graphics in web apps.
Truly a ruler of rulers.