RightInbox Adds Reminders to Gmail and Notifies You When Email You Sent Is Read

Firefox/Chrome: RightInbox is an awesome tool for boosting Gmail's capabilities. The extension when it first launched was designed to schedule email for sending later, but RightInbox since then has added even more useful features: email tracking, link click tracking, and email reminders.
Once you connect the add-on to your Gmail account, you'll find new buttons in Gmail for scheduling an email to be sent later, tracking an email you compose, and adding reminders to an email.