NATUZZONE  I've recently discovered Rulers and I think it's a great tool. It's the easiest way to measure everything is on the screen and take screenshots.

The interface is simple and cool. Rulers can be created and moved both thru drag and drop and shortcut (making adjustments more accurate).

I'm using it on a dual monitor configuration, with different size monitors, and it works perfectly.

There is also a Windows version available.  
(Version 2.2)