You may recall that a few weeks Amazon announced the Kindle SDK, which will allow software developers to write their own apps for the K2 and the Kindle DX. Well, not everyone has the patience to wait for Amazon.
Before I get into the code that Darron Black has running on his Kindle DX, I want to mention his first hack. The first step in hacking a device is to find an open port and connect some wires between your PC and the device. The Kindle DX has connection points for a RS232 port. Instead of running a cable between the KDX and his PC, Darron took the novel approach of installing a Bluetooth to RS232 adapter inside his KDX. (Sorry, but you can't use it to connect a bluetooth accessory.)
Darron announced that he has successfully ported the Qt framework to his Kindle DX Saturday, and he wrote plugins for for the keyboard, joystick, and the E-ink screen. He also ported a version of Sudoku as an example.
You should go read his blog post because he also provides the installer packages for Qt and Sudoku.