I like this tool:
Using Instant Eyedropper is easy. Simply move the mouse pointer to the
Instant Eyedropper icon in the system tray. Press and hold the left
mouse button and move the mouse pointer to the pixel whose colour you
want to identify. That's it. The clipboard now contains the colour
code - in HTML format (or any other format that you have previously
specified). It can be pasted and used in any text or HTML editor or
the Color Picker tool of Photoshop.