Available for both Mozilla Firefox and google Chrome, ColorZilla is an extension that can make your life easier while testing and evaluating color contrast in your web page, or choose the right color during the design process. One of its core features allows you to click (or use keyboard shortcuts) to copy colors used within the currently focused point of the page. Such colors can then be copied to the clipboard for further processing (e.g. to be analyzed within a color contrast checker), or you can further experiment with them using the advanced color picker built-in into the extension.
ColorZilla can also be used as a CSS gradient generator and a Webpage Color Analyzer (to get a color palette for any site). It can get colors of dynamic hover elements and flash objects too: seriously, are you still using flash objects? :/