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.
Komiko - Komiko is a sales intelligence solution to drive sales, retention and expansion.
Bananatag - Find out what happens to your emails after you press send! Add email tracking, attachment tracking, email scheduling and templates to your daily emails.
GMass - GMass allows you to easily schedule emails, send out mass emails and mail merge campaigns and more with Gmail! Click here to learn about more features!
GetNotify - GetNotify is a free email tracking software solution.