The Undo Send feature lets you abort the delivery of any Gmail message within moments of sending it. Undo Send can be activated from the Settings/Labs tab in Gmail.
Once the feature is activated, a new 'Undo' link will appear next to every sent email confirmation. When clicked, 'Undo' will take you back to the composing window and confirm the retrieval of the email.
However, 'Undo Send' doesn't really recall a message that has been already sent. The feature only holds emails in a queue for five seconds, giving you time for second thoughts - though some web users are already arguing that this is not long enough.
This isn't the first feature that prevents Gmail users from sending a message. Previous Labs features include the Forgotten Attachment Detector, which launches a pop up reminding you to check files are attached if the word attach or attachment is mentioned in the body text.
Google also offers a feature called Mail Googles, which asks you to answer simple math problems before sending emails, to establish you're not drunk and sending something you'll later regret.
See also: 5 best free Gmail add-ons