Khan Academy

All videos are arranged into categories, including biology, economics, arts, computing and more. There’s also a section with test preparations, one with tips for college admissions, as well as talks and interviews. You can explore the app and find interesting videos or watch content on a specific topic. Sign in with your Facebook or Google account in order to track your progress and get badges to keep you motivated.
 
Videos come with transcripts to make them easier to understand. Videos can be added to a list inside the app and downloaded for offline viewing. You can view videos full screen and adjust the playback speed, but it doesn’t have tools for changing video quality.