Works with the Classic Controller, Hori Battle Pad, NES Classic Mini Controller and most likely the SNES Classic Mini Controller.
It uses the HID-Project library and so will only work with Arduino models which are supported by it (e.g. Leonardo, Pro Micro).
If you don't want to ruin your controller's cable/plug, simply buy a cheap extension cable and cut it open to connect the pins to your Arduino.
Licensed under the MIT license. See the LICENSE file for full text.
Details about the controller port and protocol taken from http://wiibrew.org/wiki/Wiimote/Extension_Controllers