In a surprise pre-E3 announcement, Nintendo has announced "The Gamecube Controller Adapter." As the name clearly spells out, it's an adapter that allows you to use the Gamecube controller with the Wii U. Though it will provide an additional controller option for Super Smash Bros. players who aren't necessarily fans of the Wii U's Gamepad or Pro Controller, I'm assuming the Gamecube controller adapter will work with other Wii U games as well.
The announcement was made in a video created for the Super Smash Bros. Invitational which will be held during E3 on Tuesday, June 10 from 4:00 to 6:30 pm PT. Those looking to attend can find all of the details here. For those unable to attend, the event will be livestreamed on both Twitch's E3 channel and Nintendo's Twitch channel.
Oh, Gamecube controller, you beautiful, oddly shaped bastard. How I've missed you.