Free Ruler for Mac OS X

Features:
A horizontal and vertical ruler which can be used independently or simultaneously.
Measurement in pixels, inches, picas, or centimeters.
Automatic alignment of rulers.
The ability to group rulers and move them around as a unit.
Shadowless rulers with customizable transparency.
Interactive tick marks which follow the mouse location for more precise measurement.
Independent resizing of rulers, from 200 pixels to 5000 pixels in length.
Single key shortcuts for most ruler functions.
Conversion assistant to set ruler resolution based on screen size and resolution.
Unit converter utility for converting numbers between different units of measure.
Did I mention it's free?