Bluegrams Screen Ruler - A lightweight and configurable ruler tool for Windows Desktop.
On-Screen Ruler - lightweight, free, semi-transparent ruler
Pixus - Pixus is an AIR pixel ruler application, a handy tool for web and interface designers.
Measure Schmeasure - Measure Schmeasure is a simple screen ruler for measuring the width or height between pixels in any...
JR Screen Ruler - JR Screen Ruler, also known as JRuler - Free virtual ruler for your computer screen.
PMeter - PMeter is a very handy pixel meter.