Oh how I wish that Khan Academy had a native Android app. There are unofficial Khan Academy apps, as well as a sanctioned S Educate app for Samsung devices (come on Samsung, open it up for everyone), but no official support. What I really like about Khan Academy is the breadth. There are topics from basic addition and subtraction all the way to linear algebra, college level organic chemistry, art history, econ and some coding, with new topics and lectures being constantly added. The iOS app links to your Khan Academy account which give you the advancement points from watching the videos. S Educate for my Galaxy Tablet does the same. One nice thing that S Educate does is lets you take notes while you watch the videos, and then later view and edit the notes, which feels more active then just watching. This would be a great tool to help students looking for help in math and science who are already at the library. The web interface also makes a game out of learning, giving you points, badges and achievements for learning.
Another tool that should be on everyone’s device is a Wikipedia app. Wikipedia is of course the open source encyclopedia, edited by anyone. It should not be the last step for students, but it makes a great first step. Wikipedia for Android, is a great app for looking up quick facts. In a library I use it to find author biographies and bibliographies(fans are great at keeping reading orders up to date), facts about events and other quick facts for patrons.