FollowUpThen - The simplest way to schedule an email reminder
Right Inbox - Schedule Emails in Gmail To Be Sent Later
Recurrr - Send recurring emails
Boomerang for Gmail - Boomerang for Gmail is a Firefox / Chrome plugin that lets you take control of when you send and receive email messages.
Email Studio for Gmail - Send personalized emails, schedule emails directly in Gmail
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.