I made the ultimate Halo racing accessory. It's a foot pedal which operates the "w" key on a keyboard. It is easy to make, about 1/2 hour. If you are like me, you have the stuff to make it lying around already.
Equipment needed, a keyboard with a working "w" key, some wood, some screws, a hinge and some rubber bands. Got it? OK, check the URL with instructions and photos:
http://www.findvertise.com/proracepedal/Then, it's off to the races!