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.
Mailtrack.io - The best email tracking solution I've found. Free
DidTheyReadIt - DidTheyReadIt e-mails that are sent will be invisibly tracked and the instant the recipient opens the message DidTheyReadIt automatically notifies the user.
WhoReadMe - WhoReadMe is an email tracking solution that provides details on read duration, recipient location, organization name, operating system etc.