I really like this hack a lot, especially because it works with all of the things I am already using. Also, it is 100% free.
How to Track Gmail Messages with Google Analytics
Google Analytics is often used for tracking visits to web pages and mobile apps but did you know that Google’s Analytics service can also be used for tracking your outgoing email messages in Gmail. You can add the Google Analytics tracking code to your Gmail messages and you’ll instantly know if your email message has been seen or opened by the recipient.