LAST UPDATED: FEBRUARY 11TH, 2014
A huge range of topics, including biology, physics, math, economics, and more are covered by the Khan Academy, which allows you to learn almost anything for free.
There are over 4,200 videos and articles available through the app. The videos can be viewed from the app itself or downloaded individually or as a playlist for viewing at your leisure. Meanwhile, subtitles allow users to follow along and reinforce ideas, skip ahead or go back by navigating through subtitles.
Students can make use of this extensive library of educational content, and can earn a variety of badges and achievements for doing so, while parents and teachers can check what their students have been learning and doing.