Google Mail will deliver a mail even though the recipient address ends with ‘+’ and some more text, like email@example.com for example. This feature can be used to create an unlimited number of mail addresses that can be used for registration purposes. For example, say that the MSN web site requires your email address for you to download some stuff you need. You don’t want to give up your real email, since you’re afraid you’ll be spammed. Yet you need to give a valid email since they might send registration information to it, like the password. The solution is to use firstname.lastname@example.org and the mails will still be delivered to the john.doe mailbox. However, you can now quickly identify if the address you gave when you registered at MSN has been used for spam, and create a filter to handle them. GMail filters allow you to perform any or all of the following actions: archive, delete, label, forward, mark with a star.
Note that although this follows the Internet standard for email addresses (RFC 822), some sites simply ignore this and incorrectly disallows addresses with plus signs.