Not sure if this has changed in the last few years, but Khan Academy heavily invested in App Engine: <a href="http:&#x2F;&#x2F;highscalability.com&#x2F;blog&#x2F;2013&#x2F;4&#x2F;1&#x2F;khan-academy-checkbook-scaling-to-6-million-users-a-month-on.html" rel="nofollow">http:&#x2F;&#x2F;highscalability.com&#x2F;blog&#x2F;2013&#x2F;4&#x2F;1&#x2F;khan-academy-checkb...</a>