I’m pretty sure this is one idea that started as an April Fool’s joke, but new in Gmail Labs is a filter to prevent you from sending out drunken, late-night or otherwise ill-advised emails.
When you send an email during your specified time, it prompts you with various mathematical equations which you have to solve before it will send out your email. If you’re too out of it to fire up the calculator, your email will have to wait till another day.
Oh, and the one catch: your math test is timed. Takes me all the way back to third grade.
By default, the mail goggles kick in late at night on weekends, but you can change the active times once you enable the goggles, if you so desire.