I love how all the websites copying your design actually have a copyright mention at the bottom. The nerve is unbelievable! Beyond that I agree with the "obsolete your own design approach", and, however fun adding stuff to other's websites might be, that would certainly open you up to a suit, as you couldn't pretend you didn't know. That being said, if some customers paid top dollars for what is essentially your design, only not to you, that is a huge loss for you. I'd definitely try and settle with the end customers amiably in this case, or sue if they behave like assholes, and they can then sue the dev or web agency they picked themselves.