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 can schedule your MomentoMail into the future, but it seems same-day sending isn't allowed. You've got a simple queue on the left 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.
Momentomail [KyMa Labs]