I got hit by "Distributed Spam Distraction" recently.
It works by having a bot signup on thousands of websites at once with your email. The purpose is to flood your email with hundreds of welcome message emails every minute so you will miss the real security message emails (such as someone resetting your password).
What makes this attack so evil is that these are real sites you have to individually unsubscribe after the attack is over. This includes many sites from countries without email unsubscribing laws. So to this day, I still get hundreds of emails everyday from these sites who think I have signed up for their newsletter/product/etc.
I would not be against enforcing a captcha on every site out there just to prevent these kinds of attacks.