1. The simplest way to schedule an email reminder

  2. Boomerang for Gmail is a Firefox / Chrome plugin that lets you take control of when you send and receive email messages.

  3. FollowUp for email is a lightweight productivity suite that keeps your conversations and tasks from slipping through the cracks. Set perfectly timed reminders that arrive at the top of your inbox, right when you need them.

  4. Schedule Emails in Gmail To Be Sent Later

  5. Never-Forget.me is a delayed messaging service allowing you to compose email messages now which remain unsent as long as you click a link, at a recurring interval you specify, to continue to postpone the sending of your message

  6. Automatic follow up emails. Keeps on bumping until you get a reply.

  7. B

    Bumper is a stripped down version of FollowUpThen, followup.

  8. LetterMeLater.com allows you to send emails to anyone, with the ability to have them sent at any future date and time you choose.

  9. Laytr is the easiest way to defer emails.

  10. MF

    MailFred let's you temporarily archive and set reminders for GMail.

  11. Mailreminder.net is the product of some of my own frustations with trying to remember stuff during...

  12. E-mail Follow-Up Microsoft Outlook add-in can monitor sent messages which keep being unanswered.