During E3 2006, Nintendo introduced a Classic Controller, which plugs into the Wii remote via a cord in a similar fashion as the Nunchuk. The overall configuration is similar to that of other major seventh generation console controllers, and is also particularly similar to the SNES controller.
The Classic Controller features two analog sticks, a D-pad, a, b, x and y buttons, the L and R shoulder buttons, and two Z buttons (labeled ZL and ZR) next to the L and R buttons, respectively. It also has a set of -, Home, and + buttons like those on the Wii remote.
In 2009, Nintendo released a revision of the Classic Controller named the Classic Controller Pro, with a new wing grip design and the ZL and ZR buttons relocated behind the L and R buttons as secondary shoulder buttons, in the vein of the Z button on the GameCube controller. As it was released after Brawl, the game makes no reference to the new model; all images show the original Classic Controller. However, the controller is fully compatible with Brawl, and is considered superior to its original counterpart by competitive players.
3rd party controller manufacturer PDP, with official licensing from Nintendo, released a "Wired Fight Pad" controller in November 2014, a Classic Controller whose control layout has been redesigned to strongly emulate the look and feel of a Gamecube controller. However, its technical workings are just like any other Classic Controller and as such it connects to the console wirelessly by plugging into the Wii Remote. Owing to its design and release date, some players presume that it is targeted primarily towards Smash Bros. players. Another Gamecube controller-shaped Classic Controller exists on the market, the Hori Battle Pad, which is much closer to the GameCube controller than the PDP Fight Pad. 
In the Super Smash Bros. series
In Super Smash Bros. Brawl and Super Smash Bros. for Wii U, the Classic Controller can be used as a viable control scheme, along with the standalone Wii Remote, the Wii remote and Nunchuk combination, and the GameCube controller.
Standard Controls (Super Smash Bros. Brawl and Super Smash Bros. for Wii U)
Standard Controls (Super Smash Bros.)