PicPick - PicPick screen capture software enable you to grab an image on your computer screen, save, print, add effects, and share.
On-Screen Ruler - lightweight, free, semi-transparent ruler
Shottr - Shottr is a free macOS screenshot app with scrolling screenshots, OCR, annotation and measurement instruments.
Bluegrams Screen Ruler - A lightweight and configurable ruler tool for Windows Desktop.
PixelSnap - Measure every pixel on your screen with ease ✨
Pixus - Pixus is an AIR pixel ruler application, a handy tool for web and interface designers.