FutureMe.org allows you to send yourself an email...in the future! Sort of a Marty McFly move, only in reverse. I'm assuming my future self doesn't have the ability to send me email back here in the past, or I'd be getting some much-needed advice, like "Don't worry, be happy," or "Stay home sick on October 16, 2024," or "Eat more fiber, for Christ's sake!"
On a semi-related, and crazy, note, check out RaptureLetters.com, which allows you to sign up your friends and family for a free email letter notification when you disappear into heaven at the start of the End Times. Or whatever. Hmmm, I guess they employ one sinner to stay behind and run the server?