Google Labs is a place you can get nifty little apps to add on to Google pages. After a disaster of an email discussion which was accidentally sent out to a much larger audience than intended (not by me, phew), I enabled an app called "undo".
You click send, then your heart jumps up because you accidentally put someone in the To field you didn't mean to. Click undo. You have about 5 glorious seconds. Thanks, Google.