pXRay is described as 'powerful toolkit for on-screen graphics and layouts with Dimension Measure, Color Loupe, Alignment' and is an app in the Development category. There are three alternatives to pXRay for Mac and Windows. The best alternative is Red Lines Tools, which is free. Other great apps like pXRay are PixelSake (Paid) and xScope (Paid).
PixelSake is Windows App to measure distance any element image on screen. It's a tool to make Web Developer / Web Designer easier to measure things so they can save more times than just measure manually the screenshot with old way.