I love Khan Academy! This is a great way to access - a search of the iOS app store shows 57 individual Khan Academy apps covering topics from Biology and Finance to SAT-prep and Differential Equations.
If you like the Khan Academy's excellent videos and online lessons then you're probably going to like the new Khan App. The Khan App is a free app for the iPhone, the iPad, and Android-powered devices. The Khan App provides access to the same videos and lessons you can find on the Khan Academy website.