Right Inbox - Schedule Emails in Gmail To Be Sent Later
EmailPlayground - EmailPlayground provides solution to schedule and send emails with ease.
Boomerang for Gmail - Boomerang for Gmail is a Firefox / Chrome plugin that lets you take control of when you send and receive email messages.
Seventh Sense - Seventh Sense helps marketers and sales professionals reach the people that make them successful using artificial intelligence powered email.
FollowUpThen - The simplest way to schedule an email reminder
SendLater - Email scheduler for Mac Mail.