I remember reading somewhere that you could use the DS' touchscreen and stylus as a mouse replacement, using an N-Card (or clone thereof) with its Slot 2 flashing thingy: When the Slot 2 device is connected to your PC via USB, stylus movements on the touchscreen are translated into mouse movements on your PC.
I'm not sure, if you need an extra (DS) app for that though. I've always meant to get an N-Card to try this very function, but haven't gotten around to actually doing it (yet).
Granted, it's not exactly a gamepad replacment, but nifty nonetheless.