Ruler for Windows - It's a screen ruler for Windows.
Bluegrams Screen Ruler - A lightweight and configurable ruler tool for Windows Desktop.
ScreenRuler - ScreenRuler lets you measure objects on your Ubuntu desktop using six different metrics.
Meazure - What are the dimensions of that image? How big is this window?
On-Screen Ruler - lightweight, free, semi-transparent ruler
KRuler - KRuler is a screen ruler tool for measuring the size, in pixels, of items on the screen.