Mailtrack.io - The best email tracking solution I've found. Free
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.
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.
ContactMonkey - ContactMonkey tracks email opens and link clicks.
Komiko - Komiko is a sales intelligence solution to drive sales, retention and expansion.