SendRecurring - SendRecurring allows you to setup, send, and maintain scheduled email reminders.
EmailTracker.cc - Free Email Tracker for Gmail. Close More Deals with Comprehensive Email Tracking, Automated Follow-up Reminders, and Detailed Click Insights
Recurrr - Send recurring emails
Spark Mail - Spark helps you take your inbox under control. Instantly see what’s important and quickly clean up the rest. Spark for Teams allows you to create, discuss, and share email with your colleagues
FollowUpThen - The simplest way to schedule an email reminder
MailTime - Go through your email as fast and easy as text-messaging