AfterShip - AfterShip is the shipment tracking API for ecommerce businesses and marketplaces.
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.
TrackChecker Mobile - TrackChecker Mobile app features tracking of parcels and online orders, so users don’t have to worry about losing a package by tracking the parcel right on their mobile phone screen.
GetNotify - GetNotify is a free email tracking software solution.
Track24 - Track24 app assists you in tracking the packages of more than 600 international courier and postal services from all around the globe.
Mailtrack.io - The best email tracking solution I've found. Free