Inbox Creature - A super cute virtual pet, delivered right to your inbox.
SendRecurring - SendRecurring allows you to setup, send, and maintain scheduled email reminders.
Inbox Grader by Freshworks - Free metrics 📈 for your business and team email ✉️
Recurrr - Send recurring emails
Mailtrack.io - The best email tracking solution I've found. Free
FollowUpThen - The simplest way to schedule an email reminder