Boomerang for Gmail - Boomerang for Gmail is a Firefox / Chrome plugin that lets you take control of when you send and receive email messages.
Right Inbox - Schedule Emails in Gmail To Be Sent Later
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.
Never-Forget.me - Never-Forget.me is a delayed messaging service allowing you to compose email messages now which remain unsent as long as you click a link, at a recurring interval you specify, to continue to postpone the sending of your message
Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.
Prospect.io - Prospecting, like a boss. Find any email, reach out, measure everything and integrate with your CRM.