NudgeMail - Write or Forward any email to NudgeMail.
FollowUpThen - The simplest way to schedule an email reminder
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.
SendRecurring - SendRecurring allows you to setup, send, and maintain scheduled email reminders.
Laytr - Laytr is the easiest way to defer emails.
Rebump - Automatic follow up emails. Keeps on bumping until you get a reply.