Track your keyword rankings on Google and receive daily email reports, for free.
Sign up here for access where you can add domains and keywords. Once a day, you'll receive a report by email with the keywords' ranking. That's it.
You can add unlimited domains and up to 50 keywords.
My assumption is that most individuals are hobbyists keyword trackers and are not looking to pay a monthly subscription.
Therefore, SERP Inbox will always be a free product to provide value, while testing alternative revenue methods.