ReadNotify - ReadNotify enables the user to know when email is sent and the reciever reads the email.

GetNotify - GetNotify is a free email tracking software solution.

Intelliverse Email Tracker - Intelliverse Email Tracker provides insights to increase the ​productivity of your emails.

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.

Mailtrack.io - The best email tracking solution I've found. Free

iTrackmail - Want to know if people are reading the emails you send?