Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.
Followup.cc - FollowUp for email is a lightweight productivity suite that keeps your conversations and tasks from slipping through the cracks. Set perfectly timed reminders that arrive at the top of your inbox, right when you need them.
Mailtrack.io - The best email tracking solution I've found. Free
FollowUpThen - The simplest way to schedule an email reminder
Mailbutler - Mailbutler adds lacking functionality to email clients and increases your productivity
Right Inbox - Schedule Emails in Gmail To Be Sent Later