New: As featured on the frontpage of instructables.com:
Want to create your own radical controller for video games? Make the work easy with UnoJoy.
UnoJoy is a tool that lets you rapidly prototype native video game controllers for Windows, OSX, and Playstation 3. Using the popular Arduino Uno platform, UnoJoy takes care of the communication layer, allowing you to focus on novel interactivity, not USB drivers.
With the UnoJoy library, you can write your own programs to use whatever sensors you’d like to map to controller buttons and joysticks. Then, with a single click, UnoJoy turns your Arduino Uno into a native USB video game controller with support for Playstation 3, Windows, and OSX, responding to your Arduino code. The process is completely reversible, so you can easily switch back and forth between having a regular Arduino and a custom video game controller.
It includes an easy debugging application that allows you to keep the Uno in Arduino mode but see the controller state so you can quickly test your controller without having to swap firmware.
For more projects, check out the blog at http://unojoy.tumblr.com
If you want to build your own, the code and tools can be found here: http://code.google.com/p/unojoy/source/browse/