Right Inbox - Schedule Emails in Gmail To Be Sent Later
FollowUpThen - The simplest way to schedule an email reminder
Recurrr - Send recurring emails
Laytr - Laytr is the easiest way to defer emails.
Bumper - Bumper is a stripped down version of FollowUpThen, followup.
Followup.cc - FollowUp for email is a lightweight productivity suite that keeps your conversations and tasks from slipping through the cracks. Set perfectly timed reminders that arrive at the top of your inbox, right when you need them.