This past May, one of my colleagues notified me of a new tool called "Rooler" that allows you to easily measure anything displayed on your screen.
I definitely consider this tool a "must have" for anyone out there doing Web development. While tools like Firebug and the new developer tools in Internet Explorer 8 are great for analyzing the layout of Web pages, I often find myself using Rooler when tweaking CSS rules in order to achieve that "pixel perfect" design. For example, yesterday I was working on styling the new customer service portal that I'm working on and I wanted to match the customer's current .com site. Rooler enabled me to identify the dimensions of the header and margins of various elements with almost no effort whatsoever.
If you haven't already discovered Rooler, I strongly recommend you download it today.