Using magic mail addresses to guard against spam
Apparently many email systems, include gmail, support magic in the local part of an e-mail address. For example, if your mail address is mymail@gmail.com you’ll also receive mails from mymail+randomword@gmail.com.
The obvious use for this feature is some manual spam protection: When signing up for a web service you can easily include a unique address for that specific service. This might be something like mymail+twitter@gmail.com or mymail+homebanking@gmail.com. If you ever start receiving too much spam from a given service, just blacklist the unique address.