Simply Remind Me - Free email and text reminder service so you don't forget the important things in life!
Right Inbox - Schedule Emails in Gmail To Be Sent Later
FollowUpThen - The simplest way to schedule an email reminder
Recurrr - Send recurring emails
Auto Mail Sender - Auto Mail Sender Standard Edition official page. A powerful and easy-to-use eMail scheduler.
Kana Reminder - Kana Reminder is a program which can be used to set a reminder to be triggered at a specified time.