The Ruler Tool is a great little utility for accurately measuring anything on your computer screen. This includes images, font heights, dialog boxes, etc. This can be very useful for web work and graphic design - allowing you to avoid having to screen-grab and view in an image manipulation package.

The ability to make the ruler semi-transparent and then drag a line or region exactly over the area you are interested in makes lining things up extremely easy and accurate. Moreover, you can set your own custom scales and even reverse and rotate the whole ruler. Best of all, it is completely free.