SndLatr - SndLatr is a Gmail plugin that makes it easy to schedule emails to be sent automatically at a later time.

FollowUpThen - The simplest way to schedule an email reminder

EmailPlayground - EmailPlayground provides solution to schedule and send emails with ease.

Right Inbox - Schedule Emails in Gmail To Be Sent Later

RGB Technologies - Email Scheduler - Email Scheduler provides an intuitive email scheduling solution, integrated within the existing Salesforce account.

SendLater - Email scheduler for Mac Mail.