If you're in the business of pirating colors from websites, then mooColorFinder is here to help you. Just type in a web address (say, your favorite blog) and mooColorFinder will return the colors used on that website. The colors are display as little tiles of color; which you can then mouse over to get the hex color.
Now, we should note that this web service doesn't seem to always work correctly on certain websites. We believe it has to do with the way in which the site was coded.
If you prefer desktop applications to web apps, you could always use DigitalColor Meter.app (Mac; found in /Applications/Utilities/) or Instant Eyedropper for Windows.
