Now you can back date e-mails when you send them. You can even mark them as “Read” before they’re sent. So say you forgot to e-mail somebody some information? Just back date it, mark it read, send it. Then tell them to check again. You sent that two weeks ago!

http://mail.google.com/mail/help/customtime/index.html

This is the sweetest idea since Google TiSP!