“Within Gmail Labs, an option now exists under Settings to turn on “Undo Send.” From that point on, whenever an e-mail is sent, an “Undo” link will appear on the send mail confirmation. By clicking that link, the message will be routed back into composition – where the user can then change that “Meeting on Firday” to “Meeting on Friday.”
This feature can’t pull back an e-mail that’s already gone; it just holds your message for five seconds so you have a chance to hit the panic button,” Michael Leggett, user experience designer for Google, wrote on the official Gmail blog. “And don’t worry – if you close Gmail or your browser crashes in those few seconds, we’ll still send your message.”
Pretty useful feature if you ask me and I sure did need it more than once.