Khan Academy Videos
Another thread prompted me to mention an extraordinary resource for students and teachers: the "Khan Academy." Briefly, this consists of over 2600 short videos posted on YouTube covering a vast array of topics from math and science to history. The videos are short, easily digestible "lessons" dealing with individual topics. They're deceptively simple, displaying only a blackboard and the voice of a particularly gifted teacher. They're especially useful for covering "holes" in a subject where a student needs to understand a particular concept more fully but doesn't want to sit through an hour lecture waiting for the topic to be covered.
The videos can be accessed on the web (Khan Academy) with the iPad's web browser of via individual apps available free of charge in the App Store. If you aren't familiar with the story behind the Khan Academy, an excellent introduction can be found via the Ted app, as well.
If you're a student or a teacher from kindergarten through college level, you owe it to yourself to browse the amazing range of content available.
I have seen some of these and agree that the Khan Academy is a valuable resource.