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.
PixelStick - PixelStick is a tool for measuring distances and angles on the screen.
Pixus - Pixus is an AIR pixel ruler application, a handy tool for web and interface designers.
MB-Ruler - MB-Ruler measures distances and angles on the screen and distances on a map.