I was looking for a way of using the same email for multiple twitter accounts and found a link which explained that google email considers email@example.com and firstname.lastname@example.org as the same address.
Straight away that gives you a two for one deal on your google email. It doesn't end there. Google email addresses also ignore dots (.) when emails are sent to them. So email@example.com is considered the same as firstname.lastname@example.org. So all variations will end up in the same google mailbox
There are many applications for this feature including spam control however for me it makes it a lot simpler when signing up for web applications like twitter where your email address has to be unique for each account.
Hope this helps someone.