I've used my iPad for gaming, for eBook reading, for taking notes and doing art. But I haven't yet used it to enter the fourth dimension. Until now...
The Fourth Dimension is a new app for iPad and iPhone. This app is a 30-page interactive book that explains the fourth dimension and lets you directly manipulate a 4D object called a tesseract using a unique touch-based interface.
This is the closest you will ever get to holding a chunk of the fourth dimension in your hand without using dangerous, expensive, and prohibitively theoretical lab equipment.
More information, and a link to the App Store, at the app's website. Love the app's icon too.