MB-Ruler 

A tool to measure distances and angles on the desktop. The distances can be measured in pixel, cm, inch, points or twips and the angle in radiant or degree The ruler is almost transparent. Additionally it has a screen loupe and a RGB value window.
measure distance angle transparent ruler desktop point lines marker loupe remote interface

Screen Calipers Mac Edition 

Version 3.2 of Iconico.com's popular design software offers great new features to accurately and effectively measure anything on a computer screen with pixel perfect accuracy. The Screen Calipers are for the first time available the Apple Mac.
calipers caliper units distance measurement ruler measure screen custom pixels inch centimeter

Screen Calipers 

The Screen Calipers are a unique application that allows designers to measure anything on a computer screen quickly and accurately. The application resembles real-life Vernier Calipers, making the application intuitive and easier than a screen ruler.
calipers caliper units distance measurement ruler measure screen custom pixels inch centimeter