On-Screen Ruler - lightweight, free, semi-transparent ruler
JR Screen Ruler - JR Screen Ruler, also known as JRuler - Free virtual ruler for your computer screen.
Bluegrams Screen Ruler - A lightweight and configurable ruler tool for Windows Desktop.
Measure Schmeasure - Measure Schmeasure is a simple screen ruler for measuring the width or height between pixels in any...
Pixus - Pixus is an AIR pixel ruler application, a handy tool for web and interface designers.
ScreenRuler - ScreenRuler lets you measure objects on your Ubuntu desktop using six different metrics.