NudgeMail - Write or Forward any email to NudgeMail.
Right Inbox - Schedule Emails in Gmail To Be Sent Later
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.
Recurrr - Send recurring emails
FollowUpThen - The simplest way to schedule an email reminder
Rebump - Automatic follow up emails. Keeps on bumping until you get a reply.