Windows Ruler is a very helpful freeware onscreen ruler which will let you measure every object in your Windows Screen. It is a great tool for web designers and developers giving them the possibility to quickly measure everything they want on the screen where they are working.
This ruler is very easy to use, you just have to drag and drop the red ball (which represents the corner) over the object you want to measure. You can resize the ruler just with you mouse. You can show or hide the application directly from the notify icon.
You can also access the settings section of the ruler, just right-click on the icon and you can select the predefined size of the ruler or you can select the measurement units: pixels, points, inches, centimeters and millimeters. It supports all Windows versions: Windows 7, Vista, XP and 2000.