Momentomail Sends Messages from Gmail or Yahoo Accounts In the Future [Email]
You'll totally remember to send a nice email on your sister's birthday. Likewise, you won't need to remind your co-worker about that meeting 20 minutes before it happens. But just in case you're overly optimistic, you can sign into Momentomail with Gmail or Yahoo, and schedule an email for the future.
You've got a simple queue on the left side of Momentomail's page to manage or cancel your future messages, and you can have your messages BCC'ed to yourself to ensure delivery. That's about it, but that's pretty neat, under the right circumstances. It's a free service to use, and requires a Gmail or Yahoo Mail account.