This is a highly recommended plugin for designer, especially website designer. Collorzilla allows the designer to pick the color directly from the homepage, therefore, there will be no mistake in your color selection.
It is easy to use, just click on the small colorpicker icon on the bottom left of your Mozilla Firefox browser, and then move the cursor to the exact location you would like to pick the color. It will record the RGB, Web Colour Code (#xxxxxx) and the exact coordinate.
It currently support Intel Mac, FireFox and Ubuntu FC5 Linux.
If you do not have firefox, you could start:-