Mailtrack.io - The best email tracking solution I've found. Free
Mailmeteor - Send hundreds of personalized emails with Gmail.
ReadNotify - ReadNotify enables the user to know when email is sent and the reciever reads the email.
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!
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.
Email on Acid - We make the send button safer. For every campaign, every message, and every inbox, Email on Acid has the time-saving tools you need to put your best email forward.