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

Strike - Share your experience and hold businesses accountable 👊

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

dip - accountability groups for developers

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.

Wodify - Attract and retain athletes with the only all-in-one software solution that supports billing, performance, and community building.