1. You get access to thousands of free courses on math, art & humanities, computing, science, or economics.
2. The app offers quizzes, challenges and tests, complete with step-by-step hints.
Khan Academy is a leading nonprofit organization delivering free tools and online classes for everyone who wants to learn and improve.
It doesn’t matter whether you are a student, a teacher, or a parent helping your first grader, once you sign in to Khan Academy, you get access to thousands of free courses (mainly in a form of short videos) on math, art & humanities, computing, science & engineering, or economics.
The Khan Academy app for iPad and iPhone is one of the most popular in Education category. Besides videos, it offers in-depth articles in algebra, statistics, biology, microeconomics, finance, or art history, among others.

You can also find in the app lots of quizzes, challenges and tests that come complete with step-by-step hints and instant feedback.
As the app has to handle many videos, it’s good to download them while you are connected to the fast internet. You can then watch your bookmarked content easily when you are offline.
There is so much to choose from that some users find it difficult to what to start with.
This app has helped me so much in college when I didn’t understand concepts and formulas. I don’t know why people are complaining about not seeing their progress on this app or that it lacks in depth because Khan Academy isn’t supposed to be a replacement for a class.
– App Store review
Benefits: A knowledge hub with thousands of classes delivered in a friendly way; offline mode; progress synced with the user’s account.
Compatibility: iPhone, iPad.
Price: Free.