I'm still recovering from the happiness high I received yesterday thanks to Gmail's announcement of YouTube, Picasa, Flickr and Yelp previews labs. But let the happiness high continue (I'm even more elated with this news), because there's now an Undo Send lab! Probably even more useful than the Mail Goggles Lab, this new "Undo" option can save you many headaches and sleepless nights.
Once you enable the lab, you'll have a five-second window of opportunity to click "undo" before an email is sent to the addressed recipient. You will notice the undo option right above the email, but you'll have to act fast. A notification (like the one below) will appear to let you know that your email is still within your control zone.