Gmail: Google’s approach to email: Here’s Google’s April Fools Day entry for this year.
Just click “Set custom time” from the Compose view. Any email you send to the past appears in the proper chronological order in your recipient’s inbox. You can opt for it to show up read or unread by selecting the appropriate option.
Cute, but it doesn’t hold a candle to the Gmail Paper thing they did last year, which I actually fell for. They also did Google TiSP last year, which makes me think there are other goodies out there for today.