Powerful Color Tool For Firefox: ColorZilla
ColorZilla is an impressive Firefox plugin that assists web developers and graphic designers with color related tasks.
A built-in eyedopper can get the color of any pixel and a color-picker helps playing with colors instantly. Picked colors can be added to the plugin's palette viewer/editor.
Also, generated or sampled colors can be auto-copied in CSS RGB, Hex and other formats which is handy to paste them into image editing apps.
A great feature is the ability to generate the color palette of any web page automatically and seeing where they are exactly used.
Beside these color-based functions, it can also measure between any to points