Seli

ColorZilla is firefox addon that acts as an Advanced Eyedropper, ColorPicker, Page Zoomer and other colorful goodies.

With ColorZilla you can get a color reading from any point in your browser, quickly adjust this color and paste it into another program. You can Zoom the page you are viewing and measure distances between any two points on the page. The built-in palette browser allows choosing colors from pre-defined color sets and saving the most used colors in custom palettes. DOM spying features allow getting various information about DOM elements quickly and easily. And there's more...

https://addons.mozilla.org/en-US/firefox/addon/271

Ruyi

Simple but extremely useful tool for front-end web development. Just click on the icon, hover over the area you'd like to know the hex color for, and click. Saves a lot of time that used to be used by digging through CSS or copying images to a photo-editing tool.