ReadNotify - ReadNotify enables the user to know when email is sent and the reciever reads the email.
Mailtrack.io - The best email tracking solution I've found. Free
GetNotify - GetNotify is a free email tracking software solution.
EmailTracker.cc - Free Email Tracker for Gmail. Close More Deals with Comprehensive Email Tracking, Automated Follow-up Reminders, and Detailed Click Insights
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.