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

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

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

Stripe - Online payment processing for internet businesses. Stripe is a suite of payment APIs that powers commerce for online businesses of all sizes. Use Stripe’s payment platform to accept and process payments online for easy-to-use commerce solutions.

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.

dip - accountability groups for developers