http://wiiubrew.org/wiki/UsendMii
Spent a few hours toying with this thing.
Implementation is actually pretty ingenious -- program acts as a local web-server and hosts webpage that intercepts all buttons presses and stick movements (and some limited touch functionality) into key presses on PC.
Key presses should be quite enough for some emulators but with more modern games you're gonna need actual controller functionality.
If you combine it with vJoy you can emulate an actual controller. That requires quite a lot of time to set it up.
I made DMC4SE work after some tuning, but not Super Meat Boy because it requires for xbox 360 compatible controller which vJoy is not.
So i set up x360ce (which is probably a bigger hassle than other tools combined) and it kinda worked even if that was quite ridiculous: my button presses and analog movements went through USendMii to vJoy then to x360ce then to the actual game.
Eventually i made it work for Super Meat Boy, but at this point it would have been easier to just emulate keyboard button presses.
Since you're not actually connecting Gamepad to your PC -- it's a workaround using web-browser so actual console needs to be on as well.
And it's quite limited too because some buttons in your web-browser actually do some stuff, like B goes back to previous page and Y does something too. Author uses rubber band to keep ZL and ZR pressed at all times to put browser into scrolling mode where none of the buttons work and that stupid workaround works well, but it's so stupid and obviously makes triggers unusable.
And finally there is some lag too. It's in tenth of seconds but it was noticeable in SMB.
Playing Devil May Cry 4 and Super Meat Boy with a gamepad was quite an experience though.