I've been spending the last few days tweaking CSS, trying to overcome the current browser's discrepancies in order to follow the W3C recommendations as closely as possible. But as you know, it's not a piece of cake..
I used Snapz Pro to capture screen areas, and Photoshop to measure the pixels separating one rendering from another. Until I discovered xScope (thanks Enrique). I wasn't using for an hour, that I logged onto their site to purchase it. It's not only a set of rulers, you can place guides, frames, change your cursor into a crosshair, magnify selected areas of your screen or easily see the usable real estate for any screen size, browser, for Mac or Windows - a dream.
If you design, need to measure, align and inspect on-screen graphics and layouts, xScope is today's answer.