QuickLens is described as 'Inspect your UI for colors, dimensions, angles, alignments, layouts across monitors! Do all this with a suite of tools: Lens, Ruler, Frame, Guide, Monocle and Crosshair'. There are six alternatives to QuickLens for Mac and Windows. The best alternative is PixelSnap 2. It's not free, so if you're looking for a free alternative, you could try Red Lines Tools. Other great apps like QuickLens are PixelSake (Paid), xScope (Paid), EpicRuler (Paid) and Ruler (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.
EpicRuler is and app for Mac and Windows that helps you to measure any dimension or distance on your screen. It detects objects and snaps to their edges automatically. Using the snappable guidelines you can also align things quickly in any app.