Gmail has an intersting feature that allows you to append a keyword to your email address with the plus (+) character. For example, firstname.lastname@example.org will still deliver mail to email@example.com. This essentially allows you to create a unique email address whenever you need one.
This comes in really handy is if you are ever required to enter your email into a form online. If you use a unique keyword for every form that requires an email address, you’ll be able to track down the culprit if you receive spam, just by looking at the To: header. You can then filter by that address to dump further incoming messages into the bin.
On the downside, this is just another arms race. It’s not exactly rocket science removing the +keyword from gmail.com email addresses… On the plus side, it’s useful for automatically categorizing and filtering the legitimate mail too. [via]