Mailtrack.io - The best email tracking solution I've found. Free
ReadNotify - ReadNotify enables the user to know when email is sent and the reciever reads the email.
Unlimited Email Tracker - Track opens and clicks, set reminders and delays in Gmail
GetNotify - GetNotify is a free email tracking software solution.
MailTracker - MailTracker is a mobile email tracking solution.
WhoReadMe - WhoReadMe is an email tracking solution that provides details on read duration, recipient location, organization name, operating system etc.