New Gmail Lab Feature: Cancel Send
This new lab features lets you cancel a message send. But you have to be quick, as the "cancel" link lasts only a few seconds.
From the official Gmail Blog:
This feature can't pull back an email that's already gone; it just holds your message for five seconds so you have a chance to hit the panic button. And don't worry – if you close Gmail or your browser crashes in those few seconds, we'll still send your message.