Boomerang for Gmail - Boomerang for Gmail is a Firefox / Chrome plugin that lets you take control of when you send and receive email messages.

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.

Prospect.io - Prospecting, like a boss. Find any email, reach out, measure everything and integrate with your CRM.

Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.

FollowUpThen - The simplest way to schedule an email reminder

Saleshandy - Saleshandy is a cold email outreach platform that enables marketing and sales teams to scale their email outreach operations seamlessly.