FollowUpThen - The simplest way to schedule an email reminder
Boomerang Respondable - Write perfect emails with Artificial Intelligence
Right Inbox - Schedule Emails in Gmail To Be Sent Later
Mailbutler - Mailbutler adds lacking functionality to email clients and increases your productivity
Boomerang for Gmail - Boomerang for Gmail is a Firefox / Chrome plugin that lets you take control of when you send and receive email messages.
Wordzen - We write your emails for you. Literally.