PicPick - PicPick screen capture software enable you to grab an image on your computer screen, save, print, add effects, and share.
On-Screen Ruler - lightweight, free, semi-transparent ruler
Meazure - What are the dimensions of that image? How big is this window?
Pixus - Pixus is an AIR pixel ruler application, a handy tool for web and interface designers.
PMeter - PMeter is a very handy pixel meter.
KRuler - KRuler is a screen ruler tool for measuring the size, in pixels, of items on the screen.