In a past life as a graphic and web designer, I often needed to know the pixal measurement of images on the screen. If you have ever had a blog, wiki or other website, you will know that sometime size really does matter.
I used to use a pixal rule app like the ones below. It sits above everything else, can measure horzontal and vertical in pixels, inches, picas, or centimeters. They are free and come with a whole lot of other good features.
The thing is I am really lazy and opening a new application is sometimes just too much! After some googling and luck I found a discussion on Mac OS X Hints, on measuring screen elements in pixels. It turns out to be the simplest and effective way of measuring pixels on Apple Macs.
Use the built-in screen capture command (Cmd-Shift-4) to drag and measure the area you want to measure. Click and drag to measure it; to avoid taking a screenshot press the Escape key before letting go of the mouse button.
I use this when embedding forms, videos, other webpages, polls etc. into tables when I have limited area on my webpage.
I'd love to know what use you find for it.