Window Ruler is exactly what it says it is - and application for measuring windows. This is more likely to be useful to web developers than anyone.
Installation is easy, and you can keep it running in the background with a little icon in the system tray should you need it. Window Ruler can be used in two ways. Firstly, you can drag the horizontal and vertical rulers to the length you require, or secondly you can drag the rulers over a window with the red button, and it will automatically adjust to the size of the window.
You can get a precise measurement for your window in pixels, points, centimeters, millimeters or inches. There are very few other options available in Window Ruler, except choosing to hide it or not, and changing screen resolutions.
It's not possible to change the angle of the rulers, in case you want to measure something diagonal, and using it on more than one monitor is a little buggy. It has a habit of snapping out of view, and need to be restarted to work again.
Window Ruler is of limited use, suffers from slightly idiosyncratic design and is neither attractive nor user friendly.