How often have you wondered what the pixel measurement of a given image or space was on a web page? How wide is that column? How much room do I have for a banner? Kevin A. Freitas has created a brilliant add-on that works with Firefox, Chrome, and Safari called MeasureIt. Once installed, you’ll see a small ruler-like icon in the upper right corner of the browser window (this screenshot is in Chrome).
Click on it, and you get a crosshairs cursor that lets you measure anywhere within the browser window. Figuring out web page measurements used to be a pain. If I wanted to know the size of an image, I’d copy the image and open it in an image editing program to check. If I wanted to know the size of an available space, I’d make an educated guess and do trial and error until I got it right. Those days are gone, thankfully :)