When signing up for online services or newsletters, most of us feel some level of anxiety about handing over our closely-guarded e-mail address to an unknown and potentially untrustworthy third party. To work around this problem, some create "throw-away" e-mail addresses that can given out indiscriminately, but having to deal with multiple e-mail addresses can be a significant headache.
Gmail (and Google Apps) offers support for a more convenient solution to this problem: an instant "throwaway" e-mail address can be created by appending a "+" sign, followed by any combination of words or numbers, to your e-mail address. E-mails sent to this address will still be delivered to your inbox, but will retain information about the special '+' address they were sent to.
How is this useful? Imagine you signup for a service like Twitter, but want to filter out any Twitter-related e-mails automatically. If your e-mail address was firstname.lastname@example.org, you would signup for Twitter using email@example.com, and any e-mails you received from Twitter would be delivered to your inbox, but would be marked as being sent to firstname.lastname@example.org.
Take another example: the popular ABA Solosez listserv. Because of the high volume of mail on this listserv, many of its subscribers use a dedicated e-mail address to avoid commingling listserv mail with "real" mail. Using Gmail's "+" notation, however, one could sign up with email@example.com, and apply a filter in Gmail to automatically label and segregate mail sent to firstname.lastname@example.org.
Finally, a significant benefit of using Gmail's '+' notation is the ability to automatically delete messages sent to an address that is being spammed or otherwise abused by an online service; for example, if you receive too many spammy e-mails at "email@example.com", you can easily set up a filter to delete messages sent to that address.
Gmail's '+' notation delivers the benefit of easy-to-create "throwaway" e-mail accounts, but without the headache of managing multiple inboxes.