Mailtrack.io - The best email tracking solution I've found. Free

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ReadNotify - ReadNotify enables the user to know when email is sent and the reciever reads the email.

Clari - Clari is a predictive analytics platform for sales people.

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.

Drift - A messaging app that helps you grow your business.