This is one of those add-ons that I couldn’t live without. It gets used every day.
What ColorZilla does is allows you to sample any point on the browser and then it gives you the reading on the bottom status bar. Not only does it give you the color code but it gives it to you in 5 different formulas. I use this exclusively with firebug when I’m doing on-screen edits to see what my changes will look like on the site.
You can read more about ColorZilla and download it at ColorZilla.com