Gmail way of tracking who sent the e-mail

Cali Lewis sent me this information and I thought it was important to share.
Take a look at this blog entry: Google Operating System: Delete All Spam In Gmail. The most important feature is not how to delete the Spam, but instead how to add "+something" to your e-mail account when you give it out to someone else. This allows you to track who has sold your e-mail address to Spammers.
For instance, when you sign up to amazon use youremail+amazon@gmail.com. If Amazon sells this address you will know where it's coming from.
I'm now renaming all my accounts like that so I can see who's the company selling my info away.