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.

Rainmaker - Rainmaker allows you to make your Google Contacts better.

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.

Heroes - Fast-moving, Nibbles-like arcade game with retro-style graphics.