I have a separate username and password for every single site I visit. I do this because I own numerous domains from which I allocate a site specific email address which in turn allows me to track which website leaked or sold my email when I get a spam. Even if I didn't do this, I would still need some kind of system to keep track of the million different usernames that I've had to come up with because of my normal name being taken, or some length restriction.
