Android: Khan Academy is a great resource for learning a variety of topics. Now, the free online education resource has come to Android, in an opt-in beta.
The beta app comes with Khan Academy's huge library of video lessons. You can also use it to track your progress, and read lesson transcripts. It's not complete yet, though. There are no practice exercises included in the app, but it's still handy for on-the-go lectures. To download the app below, you'll need to sign up for the beta here.
Khan Academy [Google Play Store]