FollowUpThen - The simplest way to schedule an email reminder
Right Inbox - Schedule Emails in Gmail To Be Sent Later
SendRecurring - SendRecurring allows you to setup, send, and maintain scheduled email reminders.
Boomerang for Gmail - Boomerang for Gmail is a Firefox / Chrome plugin that lets you take control of when you send and receive email messages.
GMass - GMass allows you to easily schedule emails, send out mass emails and mail merge campaigns and more with Gmail! Click here to learn about more features!
Email Studio for Gmail - Send personalized emails, schedule emails directly in Gmail