iPad: The Khan Academy provides over 3000 free educational videos, exercises and lectures covering the span of everything from maths, physics, finance and history. It's one of our favourite places to get a free education online, and now its complete library is available on-the-go on your iPad.
The app includes downloadable videos for your offline educational needs, searchable subtitles, progress tracking (so you can pick up where you left off from your desktop) and so on. The app doesn't currently include Khan Academy's exercises, but that's slated as "coming soon".
Khan Academy [iTunes App Store]