CS2 was offered very briefly by Adobe as a download that would install without a custom license key. It was a brief loophole, I think it was intended for a group of users who for some reason could not upgrade otherwise so they offered this as a rather crude stopgap measure. The thing went viral with people claiming that Adobe offered free Photoshop (which they technically didn't) and Adobe shut it down soon after. Of course, the genie was already out of the bottle by then and the CS2 download lingers in many places. This CS2 version did not need to authenticate with an Adobe server, so the install would just work, and keeps working indefinitely.