When giving out your email to a website, change the email slightly using a + sign like this: "<youremail>+<website>@domain.com". The email will still get to you but when they share it with affiliates you'll at least know who shared it.
The only one I know of is Google. So gmail and private email hosted by Google Apps. Anyone know of other email services that work this way?
What you might be referring to is the syntax of the email address. Yes, it is syntactically correct to use a + in an email and not all email services/servers and web forms properly recognize that it is correct to use the plus.