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.
DidTheyReadIt - DidTheyReadIt e-mails that are sent will be invisibly tracked and the instant the recipient opens the message DidTheyReadIt automatically notifies the user.
Mailtrack.io - The best email tracking solution I've found. Free
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.