Laytr - Laytr is the easiest way to defer emails.
FollowUpThen - The simplest way to schedule an email reminder
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.
MailFred - MailFred let's you temporarily archive and set reminders for GMail.