Meazure - What are the dimensions of that image? How big is this window?
JR Screen Ruler - JR Screen Ruler, also known as JRuler - Free virtual ruler for your computer screen.
Bluegrams Screen Ruler - A lightweight and configurable ruler tool for Windows Desktop.
On-Screen Ruler - lightweight, free, semi-transparent ruler
Art Directors Toolkit - Art Directors Toolkit (ADT) is the “Designer's On-screen Toolset” for Artists and Designers...
Pixus - Pixus is an AIR pixel ruler application, a handy tool for web and interface designers.