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.
Onesheet CRM - The Simple CRM for Google Sheets
FollowUpThen - The simplest way to schedule an email reminder
Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.
Freshsales CRM - A full-fledged CRM for high-velocity sales teams.
SendRecurring - SendRecurring allows you to setup, send, and maintain scheduled email reminders.