If you use Gmail, this neat trick will let you know if anyone gives out your email address. When you sign up on a web site instead of entering username@gmail.com, put in username+websitecom@gmail.com. Gmail will ignore the + and everything following it, so you'll still get the email. To find out who is responsible for an email, you can click the Show Details link. You can also search for username+websitecom to see all of the messages sent to that address.
Source: Find out who gives away your email address with Gmail trick