iPad: Khan Academy has had an iPad app for a while, but it didn't offer any of the actual training exercises available on the website. A recent update changes that, and the new iPad app includes just about everything you get with the web version.
Now, on top of all the videos you could already find in the app, you'll get the exercises needed to test your brain. It's a nicely adapted experience for the iPad too, with lots of touch gestures, syncing between devices and handwriting recognition. Overall, it's a nice update that makes the app a lot more useful.
Khan Academy (free) [iTunes App Store]