This egg works on almost all versions of Photoshop and ImageReady.
Click [Ctrl] and go to Help –> About. In case of older versions, you may have to press [Ctrl] + [Alt]. This brings up a different screen, with the code name of the software. The credit list starts scrolling after a while, and if you press [Alt], the scrolling speeds up.
Note: Read the last lines of the credits carefullyâ€” theyâ€™re really interesting.