Meazure - What are the dimensions of that image? How big is this window?
PicPick - PicPick screen capture software enable you to grab an image on your computer screen, save, print, add effects, and share.
Bluegrams Screen Ruler - A lightweight and configurable ruler tool for Windows Desktop.
Shottr - Shottr is a free macOS screenshot app with scrolling screenshots, OCR, annotation and measurement instruments.
Measure Schmeasure - Measure Schmeasure is a simple screen ruler for measuring the width or height between pixels in any...
Pixus - Pixus is an AIR pixel ruler application, a handy tool for web and interface designers.