Recurrr - Send recurring emails
Right Inbox - Schedule Emails in Gmail To Be Sent Later
SndLatr - SndLatr is a Gmail plugin that makes it easy to schedule emails to be sent automatically at a later time.
SendRecurring - SendRecurring allows you to setup, send, and maintain scheduled email reminders.
RGB Technologies - Email Scheduler - Email Scheduler provides an intuitive email scheduling solution, integrated within the existing Salesforce account.
GMass - GMass allows you to easily schedule emails, send out mass emails and mail merge campaigns and more with Gmail! Click here to learn about more features!