iTunes U gives you access to a wide range of courses from leading universities and schools right on your iPhone or iPad. There's a wide range of topics that you can learn and over 500,000 free lectures, videos, books, and other resources available.
These materials come from the best academic institutes and cultural institution around the world including Stanford, Yale, MIT, Oxford, UC Berkeley, MoMA, the New York Public Library, and the Library of Congress.
This is the kind of app that could revolutionize education, especially along with iBooks (mainly on the iPad).
The app is free for both iPhone and iPad, just make sure you have iOS version 5 on your device (currently the latest version) otherwise it wont work for you.