ARTIS Screen Ruler

Screen Ruler is a desktop utility displaying translucent rulers on top of all the windows on your screen.

It provides horizontal and vertical rulers. Background may become translucent (but not the ticks). Display units: pixels, inches, centimeters, and picas; ticks on top and bottom (i.e. left and right) of the ruler; rulers have no window shadow; display the position of the mouse relative to the ruler origin; anchor vertical and horizontal ruler; additional pixel-by-pixel moving with the cursor keys
-You also choose your own ruler color.