FollowUpThen - The simplest way to schedule an email reminder
Laytr - Laytr is the easiest way to defer emails.
SendRecurring - SendRecurring allows you to setup, send, and maintain scheduled email reminders.
NudgeMail - Write or Forward any email to NudgeMail.
Bumper - Bumper is a stripped down version of FollowUpThen, followup.
LetterMeLater.com - LetterMeLater.com allows you to send emails to anyone, with the ability to have them sent at any future date and time you choose.