If you ever design web sites xScope is a must have! It gives you all the tools you need to take measurements on your screen in several different ways. You can have a semi transparent cool looking pixel based resizeable ruler. You get a screen size mask, which is great to check how your layout work in different screen sizes and browsers. The magnifying glass will help you check the tiny details. You can also have guides, like in PS, but on all your screen. Resizeable boxes can help you make a rough layout while working in any application. Finally the crosshair cursor will give you that nice übergeek feeling we all enjoy.

Download xScope (1MB). It works fine in trial mode and registration is 15 bucks.