Laytr - Laytr is the easiest way to defer emails.
Onesheet CRM - The Simple CRM for Google Sheets
FollowUpThen - The simplest way to schedule an email reminder
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.
NudgeMail - Write or Forward any email to NudgeMail.
Freshsales CRM - A full-fledged CRM for high-velocity sales teams.