Via Daring Fireball, I find this incredibly useful $27 XScope application from The Iconfactory.
Created for designers by ARTIS Software & The Iconfactory, xScope is a powerful set of tools that are ideal for measuring, aligning and inspecting on-screen graphics and layouts. Quickly available via the Mac OS X menu bar, xScope's flexible tools float above desktop windows and UI elements making measuring a breeze.
xScope is really seven tools in one that will help any designer do their job faster and produce more accurate results.
Tools include such useful things as onscreen pixel rulers, guides and screen real estate area. Essential (well, not essential, but very, very useful) for any web designer.