Bluegrams Screen Ruler - A lightweight and configurable ruler tool for Windows Desktop.
On-Screen Ruler - lightweight, free, semi-transparent ruler
Meazure - What are the dimensions of that image? How big is this window?
ScreenRuler - ScreenRuler lets you measure objects on your Ubuntu desktop using six different metrics.
PixelStick - PixelStick is a tool for measuring distances and angles on the screen.
Pixel Ruler for Android - Essential tool for every Android, iOS and web developer.