I initially bought an Xbox 360 controller to use with emulators. I found out that not only does it perform the task of mimicking an SNES controller admirably, but with a free program you can use it in place of a traditional mouse. While this might not seem practical, it is handy when you don't have a suitable surface to use a regular mouse on. For example, when the computer you need to operate is hooked up to the TV in your living room running a bunch of emulators. Using a mouse on a couch is harder than you'd think. Also, you can actually navigate Windows more easily by assigning common keyboard shortcuts (such as Alt + F4) to buttons. It's also just a quick, easy project to waste 15 minutes doing.
If this sounds good to you, jump to the next page to find out what you'll need.
Step 1: What You Need (Hardware)
Surprisingly enough, to use an Xbox 360 controller as a mouse on a computer you need both an Xbox 360 controller and computer. More specifically:
A computer running Windows XP or Vista. Unfortunately, the drivers Microsoft provides are not for earlier versions of Windows. The computer will need one open USB port.
An Xbox 360 wired controller. This costs $40 retail, but you should be able to find it for around $20 online. If you don't already have a wireless controller or you only have one (and want another controller for multiplayer games on your Xbox 360) then the wired controller is the better choice.
An Xbox 360 wireless controller AND an Xbox 360 wireless gaming receiver. The wireless controller retails at $60, but again online you should be able to find it for half price. The wireless receiver costs $20 pretty much wherever you buy it. This option is the way to go if you already have a wireless controller or two as it only costs $20.
Now that you have all the hardware, we move onto the software.