On-Screen Ruler - lightweight, free, semi-transparent ruler
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.
PixelStick - PixelStick is a tool for measuring distances and angles on the screen.
PMeter - PMeter is a very handy pixel meter.