On-Screen Ruler - lightweight, free, semi-transparent ruler
PixelSnap - Measure every pixel on your screen with ease ✨
Meazure - What are the dimensions of that image? How big is this window?
xScope - xScope is an excellent utility that is used to measure the graphic layouts or any application’s dimensions for checking the operating systems’ compatibility with the desktop screen.
Bluegrams Screen Ruler - A lightweight and configurable ruler tool for Windows Desktop.
QuickLens - Inspect the UI of apps, illustrations & websites like a pro