IQScreenRuler is an open source app from Iftekar Quarashi that provides a pixel perfect ruler to aid in the perfect positioning of any interface elements.
IQScreenRuler provides a 4 side ruler, a free hand ruler that can be rotated, the ability to change the scale factor, and aspect ratio, and more.
This video shows IQScreenRuler in action:
You can find IQScreenRuler on Github here.
You can download the Screen Ruler App on the App store.
A great library to help QA engineers make sure that the interface is laid out perfectly.