Following in the footsteps of breathalizer phones is this Gmail add-on from The Official Gmail Blog called Mail Goggles. When you enable Mail Goggles, it will check that you're really sure you want to send that 4 A.M. email to your ex-girlfriend. And what better way to check than by making you solve a few simple math problems after you click send to verify you're in the right state of mind?
By default, Mail Goggles is only active late night on the weekend as that is the time you're most likely to need it. Once enabled, you can adjust when it's active in the General settings.
The whole thing reeks a little bit of an April Fools Joke like last year's "Unsend-your-Gmails" gag. But since it's October, we'll have to just go with this one.