Gmail Labs has introduced a new feature today that helps you to not to send emails when you are drunk or tired. The feature is called Goggles which is active on weekend late nights by default.
Gmail Goggles can activated from Gmail -> Settings -> Labs. Once activated it shows a random math questions when you try to send a mail in weekend late nights. You can change the time and day for the Goggles activation from Settings -> General Tab.
I may not use it much myself but it is definitely useful for people and is fun.