NudgeMail works in one of two ways. You can either forward the message you want re-delivered to you to an address that corresponds with the time you'd like it back: "[email protected]", "[email protected]", "[email protected]" all should work. Or you can write your own note to [email protected] and use the subject to set the date, then fill out the body text as you wish.
When NudgeMail responds, it sends you back your message with a header informing you of when you set the reminder, along with helpful "snooze" options (seen near the top of this post). There are options to clear your history, set emails to plain text, and otherwise fix your account—all handled through email (see Advanced Commands on the "how" page.
Nudgemail is a free service that just launched yesterday. I saw a bit of delay in activating my first Nudgemail message last night, but the service is functioning fine now for my reminders. You can enter your email at the web site to get going, or simply start forwarding your stuff to a time-sensitive address or subject at NudgeMail.