Make your own XBox to USB adapter
The xbox controller is just a USB hub and controller with a proprietary connector. With basic soldering skills, and an extra USB cable, you can use your the controller with your computer, for free.
Option 1 (dirty) You can do a dirty hack and slash job by just cutting the wires then soldering them together matching the colored wires. Then use non conductive(electrical) tape to prevent electrical shorts and protect your solder joints. Note: The xbox controller has an extra yellow wire that doesn't get used.
Option 2 (clean) Take the time and split open the proprietary connector on the xbox controller. It takes about the same amount of time as applying the tape in option 1. Then solder the wires matching the colors and use your soldering iron to make a small hole for the new USB wire. This makes for a very neat and clean modification and you can still use the controller with your xbox. Pictures and more info
You can also buy adapters that do the same thing from many places online. Here is an example, but you can find many more by using a Google Search for xbox usb adapter