EmailPlayground - EmailPlayground provides solution to schedule and send emails with ease.
Right Inbox - Schedule Emails in Gmail To Be Sent Later
Seventh Sense - Seventh Sense helps marketers and sales professionals reach the people that make them successful using artificial intelligence powered email.
SendLater - Email scheduler for Mac Mail.
SndLatr - SndLatr is a Gmail plugin that makes it easy to schedule emails to be sent automatically at a later time.
LetterMeLater.com - LetterMeLater.com allows you to send emails to anyone, with the ability to have them sent at any future date and time you choose.