FollowUpThen - The simplest way to schedule an email reminder
SendLater - Email scheduler for Mac Mail.
Right Inbox - Schedule Emails in Gmail To Be Sent Later
The Top Inbox - The Top Inbox for Gmail helps to schedule emails to be sent later, set reminders in the inbox and follow up with email sequences.
Boomerang for Gmail - Boomerang for Gmail is a Firefox / Chrome plugin that lets you take control of when you send and receive email messages.
EmailPlayground - EmailPlayground provides solution to schedule and send emails with ease.