Meazure - What are the dimensions of that image? How big is this window?
KRuler - KRuler is a screen ruler tool for measuring the size, in pixels, of items on the screen.
Pixus - Pixus is an AIR pixel ruler application, a handy tool for web and interface designers.
On-Screen Ruler - lightweight, free, semi-transparent ruler
JR Screen Ruler - JR Screen Ruler, also known as JRuler - Free virtual ruler for your computer screen.
PMeter - PMeter is a very handy pixel meter.