GMail Labs has a feature to Undo Send. Basically, if you sent an email too soon, or realize that you wanted to include some more info in the email, you can cancel that email, and make required changes. This option works for only a few seconds after you have sent the email, and during that time you see an option of “Undo” in your Gmail window.

This option works by basically delaying sending your email for a few seconds after you have hit the “Send” button. I personally find this option very useful, as many a times I forget to attach some file, or realize a spelling mistake, right after hitting the Send button. This “Undo Send” feature now gives opportunity to cancel your sent email.

To enable this feature, go to “Labs” tab in Gmail. For that, click on gear icon in top right corner for “Options”. Select “Mail Settings” from that, and then click on “Labs” tab. In this, search for “Undo”, and you will see feature to “Undo Send”. Just click on radio button of “Enable”, and then “Save Changes”. Now, Undo Send feature will be enabled in your Gmail. Of course, it works only if you are using Gmail from the web interface, and not via some email client.

source: I♥FreeSoftware