Forbes has a free service that lets you send an email to yourself and have it arrive in 1, 3, 5, 10, or 20 years. I'm going to have my daughter write one to herself. I'll write one to myself, too.
Link (thanks, Kevin!)
Reader comment: Noah (and a bunch of other people) says: "futureme.org has been doing this for a few years, a better
implementation than forbes.
they let you choose the exact date to send the email, up to 35 years
in the future.
you can choose to let your email be displayed anonymously, it is kind
of fun to sift through people's messages to themselves."