Nintendo recently announced that they are making a Gamecube Controller Adapter for the Wii U. It will plug into two USB ports on the front of the Wii U. Right now the only game that will work with it is Super Smash for Wii U. It supports up to four Gamecube Controllers. There is also no word on whether or not it will support the Wavebird Controller.
The Gamecube Controller has always been the best way to play Super Smash and I am ecstatic that it will be coming back. I will definitely pick up one of these adapters when they are released. It would be awesome if this adapter worked with other games such as Mario Kart 8 and The Legend of Zelda Windwaker.