RGB Technologies - Email Scheduler - Email Scheduler provides an intuitive email scheduling solution, integrated within the existing Salesforce account.
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.
The Top Inbox - The Top Inbox for Gmail helps to schedule emails to be sent later, set reminders in the inbox and follow up with email sequences.