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