Look through your browser or password manager for all the accounts whose randomly generated passwords it remembers. If you're not already generating random passwords and having your browser remember them, now would be a good time to start, since you're finding and updating all your accounts anyway.

For the accounts you already have: also include "welcome", "password", and "log in"/"login"/"sign in". You might also search for mail containing your email address in the body.

Also skim through the top 1000 sites or so, to jog your memory if you have accounts with those services.