Eye Toy is the PS2's biggest success. It's basically a high-frame rate web-cam for your PS2 that lets you be part of specificly designed games.
It's a huge gimmick, much like the DS. However, unlike the DS the Eye Toy is very limited in what you can do. The best implementation i've seen of it is a fighting game.
I can see Camera's being used for input again in the future but Microphones are the future for now.