FollowUpThen - The simplest way to schedule an email reminder
Right Inbox - Schedule Emails in Gmail To Be Sent Later
Laytr - Laytr is the easiest way to defer emails.
Recurrr - Send recurring emails
Bumper - Bumper is a stripped down version of FollowUpThen, followup.
NudgeMail - Write or Forward any email to NudgeMail.