The eyedropper can be used in conjunction with the Info palette to read color values from your images.

Display the info palette, by going to the Window menu and clicking Info, then select the eyedropper and move it over your image. By default, the Info palette displays the RGB and Web Color values, as well as the X and Y coordinates of the cursor. When making a selection, the Info palette also displays the width and height of the selection. By default, the Info palette also shows Document sizes.