Meazure - What are the dimensions of that image? How big is this window?
On-Screen Ruler - lightweight, free, semi-transparent ruler
Bluegrams Screen Ruler - A lightweight and configurable ruler tool for Windows Desktop.
Meander - Measuring tool and route planning software for mac OSX
PixelStick - PixelStick is a tool for measuring distances and angles on the screen.
Pixus - Pixus is an AIR pixel ruler application, a handy tool for web and interface designers.