Google introduced a new feature in Gmail that is called Mail Goggles.  Mail Goggles makes you jump through some hoops to make sure that you really want to send the email.  For right now Mail Goggles make you answer some math questions after you hit send on the email.  By default it is only active late at night on the weekends.  When you do enable it you can adjust when it is active.  Hopefully this help keep some people having some regret the next day.

Gmail Blog Post
http://gmailblog.blogspot.com/2008/10/new-in-labs-stop-sending-mail-you-later.html