The Palette Grabber extension is indispensible to web designers, in my opinion.
The Palette Grabber, when installed, will add an "export color palette" to your Tools menu. When you select this command, it will extract a color palette based on the current web page that you are viewing and export it to a Photoshop/Illustrator file.
Awesome for getting color schemes from web sites that clients like. (did I say that out loud?) :-)