On-Screen Ruler - lightweight, free, semi-transparent ruler
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.
Meander - Measuring tool and route planning software for mac OSX
ScreenRuler - ScreenRuler lets you measure objects on your Ubuntu desktop using six different metrics.