Right Inbox - Schedule Emails in Gmail To Be Sent Later
Boomerang for Gmail - Boomerang for Gmail is a Firefox / Chrome plugin that lets you take control of when you send and receive email messages.
SendLater - Email scheduler for Mac Mail.
FollowUpThen - The simplest way to schedule an email reminder
SndLatr - SndLatr is a Gmail plugin that makes it easy to schedule emails to be sent automatically at a later time.
SendRecurring - SendRecurring allows you to setup, send, and maintain scheduled email reminders.