FollowUpThen - The simplest way to schedule an email reminder
Right Inbox - Schedule Emails in Gmail To Be Sent Later
LetterMeLater.com - LetterMeLater.com allows you to send emails to anyone, with the ability to have them sent at any future date and time you choose.
Recurrr - Send recurring emails
NudgeMail - Write or Forward any email to NudgeMail.
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.