Google Labs recently started a new feature known as Mail Goggles which helps you not to send emails when you are drunk or tired ! Sounds interesting !
How to enable this service : To activate this service go to Gmail>> Settings >> Labs and enable Mail Goggles and click Save Changes.
Now go to General tab and set your date, time and days and again click on Save Changes.
About this service : Well this service will be a fun to use. After enabling this service whenever you will try to send any mail you will randomly get some mathematics sums to solve.
What's the use of this service : According to me the best use of this service will be to drinkers. Its impossible to send any mail when this service is enabled.
Source : Official Gmail Blog