FutureMe is a basic EtotheF site. You can send an email as far as almost 30 years from now. This is great if you want to say hi to yourself from the past, if you are making a bet with someone you are sure you will forget, or for any other reason you can think of.
Just discovered this today. This site allows you to do as much as FutureMe, but you have the option of not having to log-in through their site. You can actually send an email from whatever service you use and it will let you send something to the future. This can be great for those late night emails you want to send but don't want to appear either weird, drunk or a loser.
As an aside, it would make life much simpler if GMail just had a "Send Later" option. Until then, these sites work great.