How to use
Bring forward the usual Color panel. (This may appear as "Show Colors" in various menus, or may be brought up when you click a color for editing.)
Click the Hex Color Picker icon (a # mark on a blue-tinted circle) at the top of the window.
Use the text field to type in or copy the hex code for the current color, or use the standard Color panel facilities (including other pickers) to tune the color to what you want.
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Tool I've found useful for years - I think it's much nicer to have a system-level color identifier than using the vague and patchy eyedroppers in Photoshop, firebug, and installing more add-ons for other programs.