Hack let you convert encrypted Mobipocket files that were intended for another reader so that that work on Kindle.
We've known for some time already that Amazon's AZW files are actually Mobi files, but Amazon didn't share Kindle's Mobi PID which would allow one to buy encrypted Mobi books for Kindle.
Plenty of free e-books in Mobipocket format show up fine on Kindle.
Amazon can do a lot of money only from selling kindle, like Apple does with the iPod.
A revolutionary portable reader that wirelessly downloads books, newspapers, magazines and blogs to a crisp, high-resolution electronic paper display that looks and reads like real paper, even in bright sunlight.