Meazure - What are the dimensions of that image? How big is this window?
On-Screen Ruler - lightweight, free, semi-transparent ruler
Measure Schmeasure - Measure Schmeasure is a simple screen ruler for measuring the width or height between pixels in any...
Bluegrams Screen Ruler - A lightweight and configurable ruler tool for Windows Desktop.
Meander - Measuring tool and route planning software for mac OSX
PMeter - PMeter is a very handy pixel meter.