Meazure - What are the dimensions of that image? How big is this window?
PMeter - PMeter is a very handy pixel meter.
Pixus - Pixus is an AIR pixel ruler application, a handy tool for web and interface designers.
On-Screen Ruler - lightweight, free, semi-transparent ruler
ScreenRuler - ScreenRuler lets you measure objects on your Ubuntu desktop using six different metrics.
KRuler - KRuler is a screen ruler tool for measuring the size, in pixels, of items on the screen.