Another day, another new feature in Gmail Labs. This one could be more useful than most, as it's something you probably have a reason to use with some frequency. It's called 'Undo Send,' and as the name suggests, it lets you take back a sent email, as long as you act quickly enough.
After enabling the feature, Undo Send works much like Gmail's other 'undo' features. When you send an email, you get a message confirming it has been sent, along with a link to 'Undo.' This message lasts for 5 seconds, at which point you lose the opportunity to take it back. While that might not be much time, it's probably enough to pull back emails where you forget an attachment, forget to cc someone, or catch an obvious typo.