Rejoystick is a program that maps keypresses on your gamepad/joypad/joystick to keypresses on a keyboard when you are running X. Since it uses SDL to communicate with the gamepad device, it can work on every platform that provides SDL and an X environment. (Though it is yet to be tested on Cygwin e.g.)
What does that mean? Well, it means that whatever buttons you feel are useful to get access to on you keyboard - you can now get access to from your gamepad.
You could use it for: