Gmail Adds an "Undo Send" Feature
It's another Gmail Labs feature, so to enable it you have to go into Settings, Labs, scroll down to Undo Send and enable it.
It's the same thing as sending any other email via the Internet, though. Once it's gone, it's gone. What "Undo Send" does is hold your email for 5 seconds before sending it.
During that time, if you click the "Undo Send" link, it will not only stop the email, it will roll back the email to the point you were at in composing it before you hit "Send."
Will it save you? We tested it and it worked. Don't try to send yourself an email to test it, though. In that case the email still showed up. Sending an email to a friend's email address, and hitting "Undo Send" worked great, however.
This isn't the first Gmail Labs feature set to save you from yourself. Google earlier introduced Mail Goggles which would keep you from sending an email if you were too drunk or groggy to answer a math question.