FollowUpThen - The simplest way to schedule an email reminder
RGB Technologies - Email Scheduler - Email Scheduler provides an intuitive email scheduling solution, integrated within the existing Salesforce account.
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.
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.