Google's Mail Goggles prevented users from sending drunken emails by compelling users to do a series of math problems before the message would be sent. Microsoft Outlook allows you to recall or replace emails you've sent (in case, for example, you included the wrong attachment or sent confidential information to opposing counsel). But folks are still looking for a full-proof solution. (Microsoft's Outlook can't recall an email that has already been opened, and---embarrassingly---it tells the recipient you are trying to recall the email!)
Enter the Harvard law students. Their service, Pluto Mail, is still in the beta stage and has some drawbacks, but it may be worth a try for those who have itchy trigger fingers when it comes to sending scathing emails.