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