Meazure - What are the dimensions of that image? How big is this window?
Measure Schmeasure - Measure Schmeasure is a simple screen ruler for measuring the width or height between pixels in any...
On-Screen Ruler - lightweight, free, semi-transparent ruler
JR Screen Ruler Pro - JR Screen Ruler Pro (aka JRuler Pro) is a great tool to accurately measure anything on your screen.
Bluegrams Screen Ruler - A lightweight and configurable ruler tool for Windows Desktop.
Meander - Measuring tool and route planning software for mac OSX