Here are two free utilities that are very helpful if you do computer graphics, desktop publishing or web design. One for Mac and one for Windows.

This software allows you to create a set of rulers on your whole screen, like most common graphic editors that allow rulers on their working areas only. Using the guide lines enables you to measure and position objects with precision.

Mac version

Freeruler

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?

ARTIS Screen Ruler

Translucent rulers
horizontal and vertical rulers
background may become translucent (but not the ticks) - diplay units: pixels, inches, centimeters, and picas - ticks on top and bottom (i.e. left and right) of the ruler - rulers have no window shadow - display the position of the mouse relative to the ruler origin - anchor vertical and horizontal ruler - additional pixel-by-pixel moving with the cursor keys - choose your own ruler color

Windows Version

Jruler
This Screen Ruler is a great tool that allows you to accurately measure anything on your screen. This is especially useful when measuring graphics, web page browser sizes or whatever. Screen Ruler can display Pixels, Inches, Picas or Centimeters.

With A Ruler for Windows you have your choice of a rich wood grain, stainless steel, clear plastic see-through or yellow construction ruler.
 
A Ruler for Windows lets you layer one or more rulers on your screen at once.  
The ruler can be flipped horizontally or vertically, resized to an exact length or resized by dragging it to a desired length.

The numbering can be reversed and the ruler length can be toggled on and off.

Ticks can be placed on which ever long edge you prefer. You can click on a tick to have a measuring line automatically drawn at an exact location.

Special 2x, 3x, and 4x modes allow you to take a snapshot of your primary screen and magnify it to more easily measure between very fine points.

As you can see through the clear plastic ruler you can even continue typing under it.