Screen Ruler: Measure anything on your screen. Do you need a way to measure the distance between two points on your screen? To get the exact location and size of an element on a web page? To measure in pixels, centimeters, millimeters, points, inches or your own units? If so -- Screen Ruler is your choice:

measure in pixels, millimeters, centimeters, points, inches define your own units horizontal and vertical guides rotation and "diagonal" measurement supported use figures to measure area and perimeter works with multi-display systems highly customizable (incl. "screen calibration") works on Windows and Mac