Hedir.com has published the following trick in their web directory that shows Gmail users how to find out which sites give away their email addresses to spammers.
When you give your email address to a website, you hope that they don’t sell or trade your address to a bunch of spammers. Well if they do, here is a simple way to see what sites are responsible for what particular piece of email. This requires you have a Gmail account.
If your Gmail login name was firstname.lastname@example.org and you went to samplesite.com to fill out a registration form, instead of just entering email@example.com as your email, enter it as firstname.lastname@example.org instead. When Gmail sees a “+” in an email address, it uses all the characters to the left of the plus sign to know who to send it to. In this example it would still send it to email@example.com.
Now whats cool is if you search Gmail for username+samplesitecom, you will see all messages that were sent to that email address.
To see who is responsible for sending a specific message click the Show Details link and you will see the complete address.