The d-pad on the Xbox 360 controller leaves a lot to be desired. More often than not, the d-pad on my Xbox 360 controller would select the wrong direction, or become completely unresponsive to one direction in particular. This modification by walletandgromice on Instructables alleviates some of those issues.
As you can see by the photo, the controller on top is unmodified, while the controller on the bottom is modified. Notice the different in the plastic bezel around the d-pad. The modified version has substantially less plastic allowing more room to move, and reducing the likelihood of contacting the bezel when selecting a direction.
Thus far I am very happy with this modification. I would highly recommend it to anyone that is frustrated with the performance of the d-pad on their Xbox 360 controller.