On-Screen Ruler - lightweight, free, semi-transparent ruler
Bluegrams Screen Ruler - A lightweight and configurable ruler tool for Windows Desktop.
Meazure - What are the dimensions of that image? How big is this window?
PixelStick - PixelStick is a tool for measuring distances and angles on the screen.
PMeter - PMeter is a very handy pixel meter.
MB-Ruler - MB-Ruler measures distances and angles on the screen and distances on a map.