After six years of beta testing, Gmail's Undo Send feature is now available for all.
Using Undo Send, you can "unsend" e-mails up to ten seconds after you've hit the send button. For those that have the option enabled, Google will delay sending the mail until after the ten-second cancellation window. If you want to extend the window to recover your messages, you can do so by up to 30 seconds.
To enable the service, head into settings in Gmail, and look for the Undo Send option in the General tab. Users beta testing the feature through Gmail Labs will have it enabled by default.