Undo for Emails : Gmail’s new Lab feature may save your day

Turn on Undo Send in Gmail Labs under Settings, and you’ll see a new “Undo” link on every sent mail confirmation. Click “Undo,” and we’ll grab the message before it’s sent and take you right back to compose.

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.

Source: http://gmailblog.blogspot.com/2009/03/new-in-labs-undo-send.html